tube ringing?

Hi folks, recently I replaced my Accuphase preamp with a CAT SL-1 Signature Mk III. My power amplifier is a Gruensch CSE II se and speakers are Soundlab A-1's. Speaker cable is Cardas Golden Cross. First I have to say that I hear a significant improvement over the Accuphase and I think it is a matter of pre/poweramp matching. The sound is quite good now but... with female voices for example Barbara Streisand I hear that the 600-800Hz region is emphasized. It reminds me of some ringing. Could this be ringing of the tubes inside the CAT? Or is it just room acoustics thing?

If you tubes are microphonic and ringing it should be obvious. One idea is to listen to a CD at a level high enough to induce the problem and then pause and listen for a risidual ringing. If you have dead silence right after the pause than your tubes are probably okay.

Try Herbie's tube dampers on the preamp tubes. Cheap & effective. Cheers,

Is this a hollow sound to the voices like they are in a tunnel ? If you cup your hands say and hold the word hello then then bring your hands together from each side of your mouth does this create a similar sound? If so I had the same problem and started moving my speakers away from the back wall until I felt the voices sounded natural.
FWIW I don't think microphonics in tubes is going to cause a 'ringing' in the 600 to 800 hz range. It would be much more noticible in frequencies above 2000hz IMHO.

Sounds like you may just be hearing the difference between the sonic nature of some tube pre-amp's which can be a tad more forward in the mid-range and quality ss which are usually much more linear. Also, consider that tube selection is critical both tonally and clarity wise. You may just need some different tubes in the pre-amp.
I think Newbee pretty much nailed it.
What may help, and I would experiment with it first, is a different speaker positioning. Play with speaker positioning a bit. May be you'll arrive at a final speaker location where you will minimize that sonic effect. Or you can research other tubes for this preamp
That is not tube ringing
Dazzdax I took Newbee's and Audphile 1's advice and changed tubes in the pre and that was where the bulk of my problem was, Valvo ECC88 replaced by Amperex US PQ 6922. This changed the nature of the sound not just reducing a unnatural "Slightly thin sounding vocals with more air than body". Thank you both it was the proper diagnosis in my case!!!
Sam, you're welcome!