Gain level on a REL sub

I appreciate all the help and recommendations. All of you have valid points and can't disagree. However, let me reiterate the situation I am having. With the Hsu sub I had had more of a "presences" feel to bass drum and bass guitar but still without the boomy and muddy sound I don't like. Perhaps maybe the Hsu sub was ported and has a lower response than the REL that I am use to. In the contrary, the Rel just disappears but in a good way. I feel it producing the full audio spectrum and not a sub/sat combo even though it is. 

I am actually using the high level connection and it wired properly. red to R, yellow to L and black to common ground on the amp chassis based on REL recommendation. As for the phase switch I have it 180. I have data to proof they are phase aligned. All you have to do is read phase traces on a graph interpret the slope angle. Again, I use Rational Acoustics for this since I have it available.

As of alot of you mention if one of the option was a defective sub. I really don't see or hear any defective issue with the sub. Maybe the gain staging in the Rel amp is not high enough but still is sufficient for my needs. 

Overall conclusion to this is that REL sub is sufficiently great as long as it produces clean, musical bass with detail and definition. 

@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)"]]