Gain level on a REL sub

@highend64 However, the gain is almost all the way up and I don feel that punch and slam I am looking for.

All REL sub-bass speakers are dramatically rolled off -6dB@rated output. Your HSU STF-2 is a -2dB@21 or 22Hz actual subwoofer which demonstrates what you're experiencing with their low frequency presentation differences. 

Not using the subwoofer crawl to properly position the HSU within your room could explain away the lack of detail you described.

Typically the rolled off nature of a sub-bass speaker simply cannot excite a rooms standing wave bass modes making room positioning far less, if at all critical. 

As an example: look at @wswright20's system photos showing his sub-bass speakers flanking the main speakers in the middle of a front wall, typically a rooms null and the worst place for actual -3dB subwoofers.

If you used the low level line connection from your preamplifier outputs with no improvement in gain I'd have the bass speaker checked out by a servicing REL dealer near you.

IMO, that extra low frequency Rythmik subwoofer option would be in play well before another over priced rolled off sub-bass speaker. 

I hope those nice folks on Addison in Berkeley are able to wiggle their way around those 52% tariffs on Chinese products. 

@highend64 I started with 1 REL T/9x and then went to a stereo pair. I don’t think you will necessarily get louder base, instead you get more of what you already hear.

Try unplugging your REL and mane sure your main speakers are in the best location to get to most articulate, fast and musical base possible. Then add in your 1 REL using the crawl method. Very small movements of the REL can make a big difference. For me, pointing the REL from near one corner, firing across the room to the other corner, was the sweet spot…..not placing the REL next to my main speakers “along the same plane.” 
 

Then add a second REL to enjoy to immense benefits of a stereo pair. 
 

By the way, I have a small room and use the high level connection with RELs upgraded Speakon cable.

hope this helps! 

BASS not base. 

@wolf_garcia BASS not base.

I make that mistake often and I'm a Double Bassist.

@labpro Then add in your 1 REL using the crawl method. Very small movements of the REL can make a big difference.

The subwoofer crawl is accomplished by positioning the subwoofer (not a sub-bass speakers) stationary at the listening position. The user then moves around the room noting and marking the strongest or best low frequency locations for subwoofer positioning.

Originally from Richard Edmund Lord and more recently from a REL brochure:

 [["REL subwoofers (I still like our original term Sub Bass System for its more complete description of what we do)"]] 

+1 on the Rythmik F12G.  I have one (one) myself and it's a great sub for music.  I have a dual system connected to it (home theater plus a 2.1 music system) and it excels in both applications.  I have no problem getting superbly musical, tight bass from it.  It's highly adjustable to that end (but does not have a DSP app associated with it - you'll have to do it manually) and digs deep.  Can't recommend this particular sub enough.