Usher cp-6381, lacking in bass?

I owned these ushers for years and they had plenty of bass. I think it's your krell amp. IMO a lot of the krell amps that I have heard don't  have a good bottom end, similar to carver amps, a lot of power but no lower end.

i did add bags of sand in that compartment below the speaker terminals on the back to tighten up the bass

" I owned these ushers for years and they had plenty of bass. I think it's your krell amp. IMO a lot of the krell amps that I have heard don't  have a good bottom end, similar to carver amps, a lot of power but no lower end."

For bass, his Krell is the strongest component. I had 2 of them. I can't be 100% sure, but I would say the preamp is the weak component.  

Now I am confused...lol
Some say the Krell is the problem, others say the preamp
Going to perform some tests today swapping components out of my ht rig like front speakers, power amp etc
Results soon......

Alright, have performed a couple of tests this morning using 2 reference cd,s . Pat Benatar, best shots and Nonzamo by IQ, both have some really good tight punch bass lines in them and soaring vocals( for rock anyway!)

First I pulled the ht fronts and swapped them in, these are sonus faber concerto grand piano and they gel perfectly in the ht rig but here with no other changes....a complete disaster!
Really muffled bass of miniscule proportions, nasal vocals and soundstage all but gone, could just be equipment matchs but also possible that in the ht rig I have the sub crossed over at 70hz so just maybe I do not even know the letdown as it is not apparent there, or maybe they just work much better with the components in the ht rig?
Whatever hooked the Ushers back up and back to near bliss.

Next I swapped out the Krell poweramp for the ht 2 channel poweramp( i have Emotiva xpa2 and xpa5 for ht duty). Different......
Not bad just ...different.....
I would say the bass was maybe a little deeper but not as tight, vocals seemed strong and sweet but the soundstage was possibly a little more compressed. Overall I think the Krell did a better job but the difference was nowhere near as drastic as the speaker swap.

I know Emotiva does not have the best rep but seriously the bang for the buck at 1300 for both amps used off eBay was not to be missed and they work extremely well in the ht rig.

Looks like the next step has to be moving the behemoth 110lb REL sub in and giving that a whirl...I am working up to that one.....

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