In my experience (I repair vintage & new Vacuum tube and Solid State amps - instrument and Hi-Fi - for last 22 years) any amp can be repaired - the problem being cost of repair vs what the amp is worth - I own Definitive Technology speakers (love them BTW) 2 x BP200TL, 1 x 3000 CLR, 2 x BP 2004 & 2 x BPX - 7 channels total - 5 of which have built in subwoofers (3 front and side surrounds) - I had a few drivers go bad, 1 subwoofer and an AMP board for sub that was DOA when purchased new. Most times with any amp that is approaching 10 years or older - the power supply filter capacitors (Electrolytics) dry out and either open or short. If they open they ususally don't take any other components with them - with the right test equipment (I highly recommend the DER DE5000 LCR meter) - you can tell which are bad either by carefully looking at them (electrolytics sometimes swell up and pop on the tops or bottoms and leak a thick gooey liquid all over the circuit board - or you can use above mentioned LCR meter and test most of them in circuit using either the capacitance setting (look at cap rating printed on the barrel of cap e.g. - 1000uf is a typical filter cap value - and measure the cap with POWER OFF UNPLUGGED for at least 30 minutes - from wall outlet and disconnect speaker wire feeds short all the terminals and the electrical plug to discharge any caps that may be holding a charge to protect meter. Set meter for 120hz frequency and read cap - if it is off by more than 20% rated value it is suspect - more than 30% (1000uf cap measuring 1500 or higher or 500 uf or lower is bad - removing one side from the board and testing again will give a more precise reading just unsolder one leg - if more than 20% higher or lower then replace - but you are looking for a cap that is way off - that's the culprit) it should be changed. U can also test ESR set frequency to 100Khz for ESR testing - should read 0.1 - 0.3 ohms - any higher cap is suspect. If reading much higher like 20 - 100 ohms cap is bad.
All this assumes u have worked on electronics and know how to use a soldering iron correctly because if not u could do more damage. These parts cost pennies as compared to buying a new board or speaker - a 1000uf cap at 300V (buy name brands either American, German or Japanese - Sprague, F&T or Nichicon or Panasonic from MOUSER Electronics, Digikey or Newark). Always buy at least rated voltage or higher if it fits - so a 1000uf @ 250V can be replaced with same capacitance at 300, 350, 500 V so long as not too big NEVER buy a cap rated at less voltage than what is called for in circuit. I usually go 25-75% higher voltage rating because the caps will run cooler and last longer and cost difference is pennies. Caps cost between under $1.00 to $5.00 each depending on size and voltage rating - Tube amp caps are more $$$ cause tube amps voltages are much higher 400-800V than solid state which are usually well under 300V (15 - 200V typical). Sometimes a bad cap, diode or resistor (inexpensive passive parts) can cause a more expensive part (transistor, FET, JFET, OPAMP, etc..) to burn out - this is a more complicated fix and those parts depending on age and manufacturer may be harder to come by.
The other option is to search Ebay - I see these speakers being sold for next to nothing - cheaper than the parts - or people parting out working DEFTECH speakers - selling just the subwoofer amp board or selling a single or pair of BP2000TLs for around $100 - then you have a whole speaker's worth of parts - or if cabinet is in better shape - substitute their speaker for yours and keep yours for parts. I am surprised Def Tech won't sell you a subwoofer board - or fix yours (by removing board and sending it to them for repair). I bought drivers from them a couple of years ago for my speakers - a couple of tweeters, one mid/bass and one 15" subwoofer.
Finally - a dumb question - did you make sure it was the sub-woofer amplifier and not the speaker that was bad? When my speaker went I couldn't easily tell which was faulty because the amp will not power up without a speaker load connected to it -- I wound up using some clip leads connecting my other subwoofer to the amp in the faulty tower to make sure the problem followed the speaker - not the amp. A quick but not 100% sure way is to totally disconnect the subwoofer and use an ohmmeter across speaker terminals - u should read around 3 ohms for a 4 ohm speaker and about 6-7 ohms for an 8 ohm speaker - if you read OPEN (infinity) then the speaker voice coil is shot not the amp. To test amp for certain - swap the know good subwoofer for the one that u think the amp is bad and see if the problem persists - the amp turns on ONLY when it receives bass frequencies & a working subwoofer is attached to the output (load) otherwise it remains off. Again - I thought I had a bad sub amp board when in fact it was a bad subwoofer speaker with open voice coil.
If you want to post some pics of the assumed bad circuit board - front and back I can take a look to see if there is anything obvious - 908 400 2760 is my cell
