FV15HP = 14-90Hz, and looks flat between 20-60
KEF sub = 30-150Hz (can't find a graph)
KEF center & sats = 65/70-55Kz (can't find a graph)
I am currently using the KHT3005SE (mostly; surround speakers are from a KEF Picoforte 1 iPod dock) and it's pretty clear that most of the commentators have not heard the set.
It's simply spectacular for the size. It also sounds better than most of the larger KEF speakers I've heard, likely due to the dispersion pattern and the careful shaping of the baffle. Being one of the better-sounding KEFs means they sounds better than most speakers period, as few firms engineer speakers as consistently excellent as KEF does. (Their Refs do sound better, and based on the measurements of the new Q900...that's the speaker to beat at anything near its price.)
Honestly, and to my dismay, they throw a better image better than do my reference LCR trio, which use expensive 12" Tannoy Dual Concentric drivers in bespoke low-diffraction cabinets.(For cost comparison, the
entire KHT3005SE set costs a bit less than 1/3 as much as
each of my reference mains. True, the big Tannoys do everything else better, with the extra output capability in the lower mids being the biggest difference.)
The basic design of the Uni-Q, along with the chosen crossover frequency, frees them from the midrange bloom that typical speakers with flush-mounted domes suffer. They're very well voiced for small-to-medium rooms. (Some listeners may wish they were voiced a bit brighter, admittedly. I don't.) Lastly, when limited to ~120Hz, get shockingly loud. Perhaps not quite loud enough to transport a listener to the Musikverein during a read of Shostakovich 10 or Mahler 8, but loud enough for most music.
That said, IMO the Rythmik sub is not well-matched to the mains, or for that matter very suitable for a modern home audio system at all. It's a bit big (so you won't really use its capabilities unless you run the bass very hot; a 12" primary sub is IMO more appropriate for 4.5" mains) and is extremely limited in bandwidth. A classic example of sacrificing something that absolutely matters (upper-bass bandwidth) for a marketing point of questionable audibility (the nth degree of low distortion in a frequency region where people are relatively insensitive to distortion.) But you already have it.
Here's what I'd do if you want to buy something new and really make a coherent system that shines on live, unamplified music and has adequate dynamics for most movies, starting with what you already have.
First, how to spend your $1500 for maximum benefit:
1) If you don't already have one, buy a decent measurement system.
It is simply impossible to set up a complex audio system without good measurements.
If you have a Mac, you can buy FuzzMeasure Pro and the recommendations on the FuzzMeasure site for mic preamps and mics. If you have a not-a-Mac, the Omnimic system from Parts Express is a good, inexpensive turnkey option. If you want a standalone, non-computer-based system, you could look for a used Velodyne SMS-1. (That's what I use. It has some drawbacks, especially if you don't also have their MIC-5 spatial averaging kit, but is the easiest one to use by far.)
2) Buy two additional small subwoofers. They can be KEF HTB2's or anything similar. If they are different, make sure the drive-units have shorting rings in their motors as the HTB2 does, because otherwise they won't play high enough. If the Rythmik were less compromised, you'd probably only need one more sub, or three total. But it is what it is. The goal is to randomize, and thus smooth out, room modes in the upper bass. (NOTE: if you can return the Rythmik or cancel the order before it ships, that wouldn't be a bad idea. Then you could pick better subs, and only need three of them total.)
3) A minDSP-in-a-Box with their 4-Way Advanced Plugin. (Google it.) There are two reasons you need a standalone EQ box. First, the HTB2 does not have onboard controls to vary its level or crossover slope, and its phase control is just a crude 0/180 switch. You need fine control of level and phase/delay to integrate multiple subwoofers. Second, ine issue is that the HTB2 is a passive radiator sub tuned well for what it is. But it will drop off in frequency. Below is the combined measurement of three subs, a DIY design with a low corner of around 20Hz and two HTB2's (5-point spatial average).
(The notch on the top of the graph is the mains' floor bounce.)
Honestly, that's probably top-thousandth-of-one-percent in-room frequency response in the upper bass. And if the HTB2 had controls for level, phase, and low-pass that would be attainable without a standalone box. But as you can see, there is more energy in the 40-80Hz octave than in the octave below. That's because there are three subs covering 40-80Hz, but only one is providing useful response in the 20-40Hz octave. However, with three parametric EQ cuts, that response becomes this response:
Now, setup:
Place the Rythmik in a room corner. Put one of the smaller subs on the opposite wall somewhere, one on the back wall somewhere, and one near the mains. Get one of them off the ground as high as you can, if you can. (If you can't, the floor-ceiling mode is the one most amenable to EQ, because it is fairly consistent across the room.)
Set your receiver's crossover to 120Hz, and play with the delay and level of the nearest small sub until the response is smoothest from 50-150Hz. The overall sub level should be lagging the mains by maybe 3dB. Then add in the next closest small sub, again using level/delay to get the smoothest response. Levels should be nearly flat now. Then add in the third. It should only add marginally to the overall level, but should smooth out peaks and troughs. Then, add in the Rythmik. Set a lowpass filter to about the F3 of the other three subs, which will be in the 30-40Hz range if they are all HTB2's. (You can use either the miniDSP or the Rythmik's built in crossover. It doesnt matter.) Starting from a low level, increase the Rythmik's level until you have smooth response down to the Rythmik's natural F3 in the room. Then apply any needed EQ for large variances, but don't sweat the narrow notches or peaks unless you later hear them. At this stage, if you want a house curve, use a shelf filter on the input side of the miniDSP to effect it. Measure to confirm operation.
