I have spent a bit of time experimenting with subs recently.
I have here a REL T5i and REL HT 1003 - both bought new. They are quite different beasts. The HT 1003 goes lower more easily, but its internal crossovers means it impacts more on the higher frequencies 200+Hz. This is OK with TV satellite type speakers where there is a big frequency hole to fill, but with Hi Fi it tramples all over the lower frequencies of the bass drivers which is not good. The T5i on the other hand has much more effective low pass filters built in and integrates very well with Hi Fi. By comparison to the HT 1003 it has less impact and depth, but it is more musical. In a medium size room it lacks the oomph of a bigger sub. With the T5i, I can barely hear any vocals when played on its own which is good from my perspective.
I've recently explored the options
1. Add a second REL T5 sub
2. Swap to a REL T7X
3. Move to BK XXLS400 FF at half the price of the T7x
I have just yesterday pushed the button on the BK sub. The RELs have 8" drivers and the BK has a 12 inch Peerless driver. It gets great reviews. Should be with me in a week.
Source
SW1X Universal Music Server UMS I Signature with Power Supply Unit PSU I Signature
SW1X USB II
SW1X DAC III Special
Audiolab 6000 CDT transport
Amps
Pre amps -- Hi fi Collective twin mono ladder stepped attenuator, with Charcroft Z-foil and silver wired. And First Watt B1 active no gain buffer.
Power amps -- Welborne 45 SET monoblocks 1.8W / Decware Taboo 6W / Elekit 300B TU-8600SVK plus further improved components 9W / ICE Power 1000W
Speakers
Highly modified Endorphin P17 open baffle speakers containing both vintage and modern alnico drivers and paper cones. All silver wired - 8" Cube Audio FC8 full range drivers and vintage 15" Altec VOTT 416 bass drivers. All sat on Townsend Audio Podium seismic isolation platforms.
BK Electronics XLS400FF Sub.
Cabling
Silver mains cables, interconnects and speaker cables by SW1X
Headphones
HRT HeadStreamer and SennHeiser HD650 headphones