did I forget to say I have a Pass Labs x250 amp?
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Dear Joeb if you're running the Audio Research LS-15 on balanced your inout imedance on the Pass X250(certainlya wonderful amplifier) will be 22 KOhms.
in Rca single ended will be 11KOhms very small and that's why the impedance missmatch you're getting no bass extention on the very bottom and possibly soem top end issues as well.
Either keep the LS15 and sell the Pass adn buy soemthing like a Audio Reserch Classic 120 or 100, or some other tubes .. or keep the PAss, and replace the preamp by something like a transistor preamp if you still have your classé preampo you might want to try it and see what it does.
my recommendation if you can find an oder T-2 or T-3 Threshold preamp, form the early 1990's they are suberb and they drive anything from 600 ohms up, also Krell and Mark Levinson preamps will drive the Pass with aplomb.
The B&W's are somewhat power hungry but 60 to 100 watts of good tubes will drive them nicely too.
Another thing is you could go all tubes, find an Audible Illusions L-2 (remote controlled thank God!!) do the wheezer mods for about $1K end up with a $6,000 sounding preamp that will beat anything short of a CAT preamp including teh AR's and CJ's, and put the rest of the money on a chinese made tube amplifier (run them with a Variac into 110 Volts very important), por the rest of the money in the bank, take a cruise, go to more concerts, etc. etc.
maybe even silver cables, you'll love those on tubes, kimber, PSC, and others will sing in that system.
well this info fits in with my sense that the music sounds and feels a bit too "compressed" if that's the right word. Although there is good bass, and pretty good clarity etc. the overall quality sounds a bit...well less dynamic than I've heard these speakers perform in other systems using a Pass Labs x250. I had heard that mixing tubes and solid state was the "cats ass" and wanted to try it, but sounds like this just isn't the best match. I am not clear on the theory behind the limitations, I wonder if someone might elaborate on this "low-impedance amp" mismatch with the tubes?
Joeb, is your LS-15 with REL caps or InfiniCaps?
I had LS-15 with REL caps and it severely lacked bottom end punch. I replaced the caps with TRT Dynamicaps and it changed the sound entirely. It had more punch, smoother and overall more natural. The difference was substantial.
I now use ARC LS-25MkI(have to say that LS-15 with Dynamicaps come extremely close to LS-25 in sound) with X250.5 into N803 and it is very good. I use ARC CD3MkII cd player. All balanced cables. By the way, in balanced configuration you will get higher output from your LS-15 into Pass amp and it may improve bass output plus you will lower the overall noise floor. ARC is meant to run balanced. Also, before giving up entirely on the LS-15, if yours is with InfiniCaps(better version) try some NOS tubes if you are using stock tubes. Sovteks or EH tubes do not have very good bass drive.
Unsound & Gonzalo have the answer. Bryston amps have the same lowish input impedance issue as Pass Labs. I don't think you'll hear compression, it'll be rolled off bass. Check out the latest Stereophile. There's a review of an $18K BAT preamp with the same problem.
The solution is to pick up a Pass Labs X1 preamp and send the tubes packing.
If you really want to stick with a tube pre, you may still be able to do so, though the options are limited. The better Sonic Frontier pre's are DC coupled low output impedance and I beleive the Atma-Sphere pre's might work too. There may be others. I have no personal experience with the Atma-Sphere, as interesting as they are, they're beyond my budget. I should warn you, that to some the Sonic Frontiers are not as tubey as most other tube pre's, still one of my favorites. Bob, offers a really good recommendation. The Pass pre's are IMHO excellent.
I use an Atma-Sphere MP1 preamp with solid state Belles 150A Reference amplifiers, which I run balanced. As Unsound guessed, the combination works very well. Likely, the MP1 would also work well with Pass amps.
The Atma preamp is not tubey (bloated midbass, rolled off highs, loose bass). It's very crisp and open sounding.