Thank you- actually the system has changed so much this year -the amp I am using right now isn’t even listed in my virtual system yet.
It has been a while since I tested tube rollin’ in the EVO400 myself, so no worries, I needed a good excuse to experience the differences again.
Quick answer: Yes, absolutely the bass tightened up with my RCA Cleartops over the Primaluna stock 12AU7’s.
I still have the Prima Luna OEM 12AU7’s in the carrier spots on the sides (#1 and 6 spots) and I left in the 1960’s Mazda/Brussells f33 rectifiers for all listening. (I think that is what they are ) And RCA Cleartops in the middle 4 spots. I listened to Buddy Guy’s - Blues Singer ’Lucy Mae Blues’ and ’I love the Life I Live’ which have always had a little loose sounding bass to my ears, and I recall it being quite bloomy in the past. I also listened to one of my favourite basstest tracks Holly Cole Train Song. Didn’t touch volume, or anything else, just turned off the pre amp, let it cool a bit, and changed tubes.
First I put in all 4 Primaluna 12AU7’s back in the middle 4 spots- Buddy Guy sounded MUCH louder, and fuller everywhere, and the bass bloomier/less tight- I had an urge to turn it down, I only lasted 5 seconds but clearly obvious-I quickly changed to Holly Cole. Surprisingly I didn’t notice this as much, I believe it was a little looser, would need to go back and forth again to be sure, but after I heard the additional sibilance in her voice it was hard to focus on the bass again!
I put in just the middle 2 RCA’s as suggested, and yes bass became much tighter and controlled- and the loudness I experienced was gone. And just as important Holly’s voice sounded less sibilant.
Putting back in the other 2 RCA’s in #2 and #5 positions to have all 4 back in front, bass was leaner again, but minimally so, perhaps half as much as the middle positions? so Warren was correct in the middle two positions made the most difference.
Now I am wondering if bass, while a little fuller and bloomier with the mix of RCA and PrimaLuna OEM’s compared to all 4 RCA’s might have been more even-keeled? Hmmm, would need more testing to determine a preference personally, but your question is answered and my curiousity satisfied.
As always, my system my ears, and this is with a Zavfino Silver Dart power cable into the back of this EVO400: YMMV- this is my result- pretty obvious actually.
And these RCA Cleartops are the only 12AU7 tubes I have experimented with so far. I planned to get more but after getting these felt the improvement was good enough, and decided to do other things first. Bugle Boys, Telefunkens may be nicer, and for your concerns tighter still in the bass, I don’t know. But Brent Jessee has these RCA Cleartops for just $39 each- so you could get all 4 for the price of one Telefunken!
Whereas a "rabbit-hole" powercord that could tighten up the bass this much would cost many multiples of this, methinks, so a tube experiment would be good bang for the buck.