Tell me exactly what you mean, though, by physically modifying the speakers? You have piqued my interest..
You can choose one of two levels of modification, based on what you are comfortable with doing/ease. Level 1 is effective and offers substantial improvement, giving you a reduced resonance cabinet. Level 2 offers substantial improvement over level 1, giving you a very low resonance cabinet.
Level 1 - Easy
1. Remove drivers.
2. Remove the acoustic batting in the speakers(
the stuff used from the factory in Primus speakers is a low density 1.5-2" thick recycled poly fibre batting -- practically useless for removing the reflections/reverberation in the frequency band of interest in the cabinet).
3. Clean walls with denatured alcohol.
4. Apply Dynamat or Dynamat-like material(Peel N' Seal from Lowe's is cheaper and just as good for this application) in layers until you get about 1/3-1/2" thick layer of this material built up on the internal walls. This material will slightly attenuate the wall resonance amplitude -- but it will substantially reduce the time decay of that resonance -- when used in the amount that I recommended. Note: Use a hair dryer or heat gun to warm up the sticky side of the material before applying the first layer. This will insure a good adhesion to the MDF walls. You do not usually need to heat the additional layers as you apply them, but do this in a warm room, and make sure the Dynamat type material is at the room temperature, not cold from outside storage.
5. Install a new, high grade acoustic damping material that will actually do the job that it is supposed to do. Use a 2" thick high density (6lb.- 8lb./ft^3) rock wool board or fiberglass board. The rock wool product is usually referred to as 8# mineral board and the fiberglass is usually called OC(Owens-Corning) 705. You will find this material at commercial fiberglass contractor/installation companies. If you can not find this material, I will gladly mail some to you for raw cost of material and shipping. Expect to pay in the $10-$20 dollar range, including shipping, if I send it to you. Cover every wall with the material, and add a 2nd layer on the rear wall in a section spanning the direct rear area of the drivers to prevent any reflection. Use 2nd layer in addition on the bottom and top wall. You can install with only 4 fasteners if you cut the pieces to fit tightly and put in the top and bottom pieces first, and then use the side pieces(fitting tightly) to hold it in place tightly. To fasten, use coarse thread drywall screws with very large diameter washers on them, and screw them through the material into the MDF at the critical points to keep the pieces from sliding loose. Use an acoustic transparent cloth over the fiberglass or rock wool board to prevent fibers from coming out of the port. Go to any fabric shop and find a loose weave cloth that you can blow through and feel the air on the other side without much restriction. You can simply tuck the cloth under the fiberglass or rock wool as you install it to hold it in.
6. Re-install crossovers, terminals and drivers
Level 2 - Difficult
1. Remove drivers.
2. Remove crossover board, terminals and wiring.
3. Look in the cabinet and locate the main bracing. Drill a 3/4" hole in the back on each side of the bracing.
4. Use a router with a high quality flush laminate bit installed. Insert into drilled holes and pull towards the outer/side walls as a guide for the laminate bit(the laminate bit has a bearing on the bottom of it to use any wall as a guide). Cut out the backs completely. Use a hammer, chisel and screwdriver to remove left over small bits of the rear wall.
5. Use hammer, chisel and screwdriver to remove all internal bracing, including the small triangular corner pieces.
6. Use large chisel and hammer(and heat gun) to remove any hot glue blobs/residue left in the cabinet.
7. Clean walls with denatured alcohol.
8. Apply Dynamat or Dynamat-like material in layers to build up to 1/4-1/3" inch. Heat the first layer before application to insure good adhesion.
9. Use a coarse(60-80 grit) sandpaper and rough up the aluminum outer layer of the Dynamat.
10. Clean the aluminum with denatured alcohol.
11. Cut 3/4" MDF to fit over the Dynamat on all walls. Have it fit snugly, but not tight(if it is tight - it might cause too much pressure on the outer wall joints over time and cause them to develop a breach at the joints). Use heavy duty Liquid Nails construction adhesive between the walls and aluminum and install the inner walls.
12. Use 1" x 2" oak wood and install bracing no farther then ever 3" in every axis direction. Cris cross the pieces and use plenty of high quality wood glue. Be certain to install bracing to anchor many points on the front baffle to rear wall. Make sure to cut an leave many points for the new rear wall to be glued and secure to the bracing.
13. Install the acoustic damping materials between the bracing(will have to cut small pieces and slide it in the matrix).
14. Cut out MDF for a new back and install the back. If you make the back solid, use at least 2 layers of 3/4' MDF laminated together. Alternatively, you could make a MDF/Dynamat/MDF construction as you did with the side and front walls.
15. Use a razor blade and straight edge and carefully remove the left over vinyl veneer on the back edges.
16. Use wood or body filler and fill in the spaces left between the new back and old back edges. Sand smooth.
17. Mask off the vinyl veneer sides. Use primer and paint to give a basic flat or satin finish to the new back.
18. Re-install crossovers, terminals and drivers.
Note: Level 2 reduces internal volume substantially. As a result the low end of the speakers is now at a higher frequency. This is not an issue if you are using subwoofers, but it would be an issue if you were using the towers as full range speakers.
Edit: Also, my subwoofer has been discontinued for quite a while now. If I were to follow through with your plan, ~ how much would you suggest I spend on each sub? And a quality crossover [and what all would I need cable-wise to integrate a crossover]?
If you can find the same sub on eBay used, get a 2nd. If not, for a low cost and decent subwoofer, I always recommend the Dayton 12" or 10" subwoofers on
www.partsexpress.com. These range from $100-$150 each, depending on if they are on sale. These are low cost, but much better than what you would expect at that price from any other vendor of which I am aware. These have good cabinets, good amplifiers and decent drivers. You can easily replace the drivers later on and re-tune the port/cabinet as an upgrade in the future, if you so felt the desire to do so.
Your receiver should have a built in crossover system -- set to about 80Hz -- set front speakers to 'small' so that they are also crossed over at the set frequency.
Either way you choose, you will spend a total of $450-$600, well under the $1000 budget and you will end up with a much better speaker system than would be possible by spending $1000. If you choose to go Level 2, it would be unlikely that you could improve much by spending $6000 for a pair of new speakers. The drivers and crossovers used on the Primus are very good -- this is not a short coming on this speaker system. The cabinet appears to be the main issue. You should take a Dremel tool with a metal cutting disc(and please wear a full face shield) and cut a line through the metal shield casing on the back of the mid drivers and remove this shell. Every Primus dirver I have encountered had a rattle originating from this shield as a result of inadequate securing measures.
Please consider using some of the saved money to build DIY acoustic panels. It will drastically increase the sound quality, based on the pictures of your room that I observed. Today, probably the easiest way to make good looking DIY panels is to buy the pre-made metal frames from Ready Acoustics. These are very nice, and are about $30 each, if I remember correctly. Just buy some 8# mineral board and use this in the frames. Get some $1/yard cloth from Wal-Mart's cloth bargain bin and you will have professional looking treatments for low $$.
BTW, here is a Primus 150 done at level 2(I also installed a real veneer - replacing the black vinyl):
-Chris
