I'm not familiar with the tube layout of the VTL 5.5, but if the 12AX7 tubes are used for the gain stage, you could sub in some 5751 tubes. These have about a 30% reduction in output.
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Here is a site that will guide you in the choices available, to reduce the gain in your preamp: (http://thetubestore.com/gainfactor.html) Whatever tubes you choose to try; be certain they have been tested, and certified, "Low Noise". Upscale Audio can provide such graded tubes(stringently tested): (http://www.upscaleaudio.com/bVacuum-Tubesb_c_38.html) Treat yourself to some NOS tubes in that pre, for an improvement in presentation.
This is from the Tube Depot Website:
One of Stevie Ray Vaughns favorite tubes was the 5751 preamp tube. With 30% less amplification than a 12AX7, but 100% interchangeable, an amp using the 5751 preamp tube creates a creamier and smoother tone than with a 12AX7. If your amp has too much gain, the 5751 is the perfect choice to "calm" the amp down and get more break-up without all the volume. We prefer the U.S. made JAN 5751 over any other 5751 available.
I have never found any version of the 5751s that I have tried to sound bright at all?
You should also be aware that not every preamp circuit will really change total gain by switching tubes. My Jolida Music Envoy barely changes gain at all when I switch from 12AX7s to 5751s. I generally like the sonics of the 5751s better, but the preamp gain stays roughly the same. No experience with the VTL, so not sure what would happen there. It's always worth a try; like me, you may like the 5751s better regardless of the gain.
I just received my 5.5 back from VTL with the V3 upgrade, and with it was a new set of o.e.m. tubes (jj tesla).
I let her burn for about 100 hours and then pulled the tubes, put in a nos set of RCA gray plates, which I prefer sonically, but was suprised to hear how much quieter they are. The back ground was nice and black with either set, but the RCA's can't be heard @ 1 foot away through my signature 185 mono's and 102db speaks.
except the 12AX7A's are now suffixed with AW. Wasn't concerned enough to see what the extra charecter meant.
The 12AT7's are just that.
I have had good luck @ audiotubes.com.
Brent Jesse has been very trustworthy to deal with. My local dealer gets run of the mill tubes but really doesn't deal with some of the older stuff.
I had tried the mullards, other clich'e makes, and a few garden varieties before settling in on the RCA's.
Being so subjective to begin with, and every other system charecteristic affecting the way they sound, it's best to do what you ar doing and sample till your hearts content.
Godd luck, and enjoy it, its a gem.
I've spent last couple of months tube rolling. I just replaced the 12AX7 input tubes and left 12AT7 alone. I tried the following.
Telefunken 12AX7 ribbed plates
RCA 12AX7 Wurlitzer branded
Philips Miniwatt 12AX7
RCA tubes are remarkable. It's must more quieter than any of the tubes I've tried. The tonal balance is more on the neutral side. It's not as tubey or mellow as others, but crispier and cleaner. There is still plenty of midrange bloom, and the tubed sound is unmistakable. Very happy with the result, especially the lowered noise floor.