Gain level on a REL sub

Well thanks for that, Larry.  Glad I could help something today.  L0L

I’m not sure if anyone mentioned this before but there is a phase switch on the back. Flip it the other way. Hope this helps 

@sns His response is right on target.

The wiring is most likely the issue. I just put 2 T/9X's in yesterday and while they were in route I verified the proper wiring scheme to use. Here is what customer service offered:

"Thank you for reaching out. Either a signal ground or earth ground will be fine for connecting the black High Level wires to, though if possible the earth ground screw would be preferred. If you experience hum from the subwoofer when connecting the black wires to this screw, then you could instead try the signal ground and see if this yields improved results. 
 
We recommend starting with both the red and yellow High Level wires connected for each subwoofer, and in most cases this will result in excellent performance and usable signal levels. However, if you find that the subwoofers have too much output, where barely raising their Level controls already results in them overwhelming the output of your main loudspeakers, then you could try disconnecting one of either the red or yellow wire for each subwoofer. This will lower the input signal gain roughly by half, and will allow for greater flexibility when setting their Level controls. The red and yellow wires are functionally identical, though we generally recommend leaving the red wire connected for the right channel subwoofer and yellow wire connected for the left channel subwoofer in these cases."

The low level may be a better option for you. I found that I have very little volume needed for twin subs but am okay with that since it needs so little (about 30% of the volume knob) to mate with the Wilsons. Took me less than 10 minutes to get a good start to proper sound. I'll further tweak it this weekend.

Lots of good tutorials on their website and manuals are also available.

@jacobsrph And yes, check the phase. If sitting next to the mains then 0 is required Put your fingers on the main woofer and sub to verify they are working in the same direction while playing.

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.