Velodyne CHT-15 replacement suggestions

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I bought my parents this sub about 3 years ago. They recently bought a Samsung LNT5271F along with a new Bello tv stand so they want something smaller to go with their new uncluttered look. I looked up the specs of the Velodyne:

SPECIFICATIONS CHT-15
Cabinet (H,W,D) 21" x 18.5" x 22.75"
Frequency Response 23Hz-120Hz +/- 3 dB
High Pass Crossover 80Hz or 100Hz
(6 dB/octave slope)
Low Pass Crossover 40Hz -120Hz (12 dB/octave, 24 dB ultimate)
Amplifier (Class A/B) 750 watts/300 watts RMS power
Woofer 15" forward firing
Magnet 70 oz.
Voice Coil 2 - 1/2" copper

What sub, smaller than this, would you recommend with equal or greater performance. It must look decent so Elemental Design is not being considered. It doesn't matter if it can be bought at a Brick & Mortar or online only. I have a SVS PCU 13 so I'm okay with Internet Direct. I don't think my parents would spend that much so budget will probably be under $600 (That takes out the small JL).

Here are some subs I found that are smaller than the Velo CHT-15. The Epik is about the same size, a little down on width, so I'm not sure of that one. Will all of these sound equal or better than the Velo. I'm not too sure if they'll lose too much going from a 15" woofer to a 10". Also, I think I read that the SB12-Plus is better geared for music than home theater. Any thoughts or comments on other subs I should look at. What about Mirage subs? Oh yeah, this will mainly be used for HT.
HSU
VTF-1
Bass Extension 25 Hz
Amp Power 200 Watts
Woofer Size 10 Inches
Dimensions 19.5"(h) x 14"(w) x 18"(d)

STF-2
Bass Extension 25 Hz
Amp Power 200 Watts
Dimensions 19.5"(h) x 14"(w) x 18"(d)
Driver Size 10 Inches

SVS
PB10-NSD
+/- 3dB from 20-100 Hz anechoic (in room extension will be 2-5 Hz deeper)
Size: 15" wide x 19" high x 21" deep

SB12-Plus
Industry leading response (+/- 3dB 22Hz-150Hz)
Size: Approx. 15” deep x 14" wide x 14 " high

Epik Valor
Enclosure dimensions: 22H x 16.5W x 21D inches (22D with grill)
Driver: Custom 15 inch extreme excursion design
Amplifier: Custom 350 watt RMS BASH amplifier

I posted this question on some other forum and a helpful person suggested: "25-31PCi in the 22hz tune." He said "I used to run a couple CHT-15. I did notice when I moved over to a 25-31CS+ that it was more of a sub than a single CHT-15. " So that along with the PCi 20-39 could be an option if my parents don't mind the Cylinder look.

Thanks for any suggestions.