Hello,
Long time Van Alstine owner here, I haven't posted for a while here in AC. I thought I'd share a little of my experience with the 550EX and tube rolling. Mine came with JJ Electronics 12AT7's and I like the sound quite a bit, very transparent, very smooth. But, I like a slightly warm sound, so I started trying out some different tubes. Siemens and Mullards made their way into the amp, and I liked both of them. At the time I was also using a Transcendence 7 preamp, and it was very good.
About 2 years ago we moved to a new home, and I lost the ability to have a separate 2 channel and HT system, so I sold my T7 and combined my 550EX into the HT system, with an Acurus Act 3 Pre/Pro. Sound quality took a big hit, to say the least. The Act 3 was so harsh that I ended up with the quite soft sounding Mullards in the 550EX in order to get it even semi-listenable. Problem was the bass was pretty woolly with that tube, so I took my Omegastar solid state amp and used it to bi-amp my VMPS RM40's. 550EX on the mids/highs, the Omegastar on the woofers. There was an output mismatch, but the adjustable pots on the RM40's allowed me to get the blend to a reasonable degree. Not perfect, but definitely more listenable than before. Unfortunately nothing could ever 'really' compensate for having that Act 3 in the system, so I just resigned myself to the mediocre sound in the name of convenience and marital harmony.
Things changed this month, I sold my Act 3 and bought another hybrid tube/ss preamp, the Kavent S-11, which is the same preamp from Audio Advisor with the Vincent SA-T1 name. I would have bought a T7 again but I really needed a remote for volume, mute, and source, especially for source, as I've got a universal remote that we use to control the system, and getting up to switch sources is NOT an option for the family.
Anyway, somewhere along the way I had forgotten I'd put Mullards in there, I thought I had the Siemens, which are much more linear and dynamic sounding, and definitely punchier in the bass. I couldn't figure out why the 550EX wasn't controlling the woofers of my RM40's with an iron fist like they had 2 years ago. I tried everything, from switching the tubes in the pre, to changing cables, moving the speakers around, re-tuning the passive radiators on the 40's, adjusting the midrange and tweeter pots around like crazy. I'd get it better or worse, but still never close to what I wanted, that transparent, dynamic, smooth, super-controlled sound I had previously.
As a last resort I opened up the amp to put the stock JJ tubes back in. Saw the Mullards and realized the Siemens were sitting in my tube drawer. In went the Siemens, did a quick tuning session on the speakers, and I'm back in bliss. I still need to do some more fine tuning of the 40's to get them dialed in super-tight, but I'm at least in the ballpark now.
Anyway, in my experience, tubes do make a difference in the Van Alstine gear, but even NOS tubes with great reputations (such as the Mullards) are not necessarily going to improve the sound of the amp. The Siemens are a good match, and the JJ's are of course good as well. I might give some other tubes a shot, but to be honest the sound of the stock JJ's and the NOS Siemens are so good that I don't feel the need to throw more cash at it, I'm pretty happy now.
