Usher cp-6381, lacking in bass?

I would definitely connect the REL sub at speaker level, rather than to the RCA outputs of the preamp. For several reasons:

1)The input impedance of the line-level (RCA) inputs of the REL sub is very probably 10K (which is the line-level input impedance of the Stentor III and many other REL subs). That is too low for the LS2 to drive without significant deep bass rolloff. ARC recommends a minimum load of 20K for the LS2 and for many of their other line stages and preamps.

2)The LS2 drives its RCA outputs with the same signal that is provided to pin 2 of its XLR output connectors. Application of a 10K load to that signal but not to the other signal in the balanced signal pair (XLR pin 3) will to some degree unbalance the signals received by the Krell amp, and very possibly affect the sonics produced by the main speakers.

3)REL recommends using speaker-level inputs where possible, so that the sonic character of the main power amp is reflected in the signals received by the sub as well as by the main speakers.

4)On the Stentor III, at least, the two RCA inputs have sensitivities that differ by 12 db, which would have to be compensated for either with an attenuator that you may not have, or by using just one of the inputs and using a y-adapter to sum the two channels externally (which is generally undesirable, IMO, and in this case would almost certainly have major adverse effects on the signals received by both the Krell amp and the sub).

Also, the suggestion George made about the LS2’s coupling caps was the first thing that occurred to me when reading your initial post. But it appears that your Krell amp has an input impedance of 100K, which is high enough to make that a non-issue.

Good luck. Regards,
-- Al

Thank you Al!
Couple of thoughts here....how best to wire with speaker terminals? Can i just piggyback another speaker cable from one set of terminals on the Krell to the sub or?
Also bear in mind this is just a test as the REL belongs in the ht rig and from what I heard from the Sonus Faber speakers this morning needs to stay there...lol.
Thoughts on a reasonable priced sub if this test helps out?
Thank you

Another option I could consider for a sub is to get a unpowered sub and a dedicated sub amp which I could feed with a line out direct out of the ls2?
Thoughts?

Yes, assuming you have the Neutrik Speakon cable that is normally used to connect the unbalanced speaker-level input of REL subs to amp outputs, and assuming the amp is not being used in bridged mode, just piggy-back the wires from that cable onto the amp's output terminals.  (Red and Yellow to right + and left + on the amp, respectively, although it wouldn't matter if those connections were interchanged, and Black to either right - or left - on the amp).

Regarding a recommendation for a new sub, the brands Soix mentioned earlier seem to generally garner favorable comments, although I have no experience with them or with other competitive models.  My only suggestion, given my earlier comments and assuming that you would want to purchase one sub rather than two, is to be sure to choose a model that provides left and right speaker-level inputs.  There are a lot of subs which don't meet those criteria.

Good luck.  Regards,
-- Al
   

Another option I could consider for a sub is to get a unpowered sub and a dedicated sub amp which I could feed with a line out direct out of the ls2?

There aren’t a great many choices of unpowered subs that are available these days, and you would have to choose an amp having very high input impedance to minimize the possibility of sonic issues resulting from item 2 in my first post above. So I suspect it would be best to go with a powered sub providing speaker-level inputs.

Regards,
-- Al