VTL Transformer Up-Grades, Worth It?

While I haven't heard the VTL Signature upgrade, I can say the the output transformer is one of the most critical items in that type of amp. It is the component that is most prone to compromising the frequency extension, and phase coherence of your music. In general, an improvement in that area is considered a major upgrade, and may also be a major upgrade in price, as good output trannies are not cheap. The push-pull designs such as VTL are not as prone to frequency response problems as are SETs, due to the center tapped layout in push-pull designs, but they will make a big difference. I would give it strong consideration.

much improved bass response. Imo worth it. I'd ask about upgrade to 450 signature status like mine were.

I own MB-185 Signatures, which already come with the upgraded transformers. While I haven't heard the older versions without the Signature tranny, I have heard their current stereo amp offerings, which also lack this feature, and have found them to sound very good in spite of the supposedly "inferior" trannies used. However, I did read a lot of what was written critically about VTL's amps over the years when researching the line, and most reviewers seemed to be unanimous in crediting the upgrade with improved sound at the time it was introduced. Luke Manley certainly promoted the Signatures as being a big step up for his company, but that could always just be marketing. BTW, are you also thinking about getting the coupling cap upgrade? I am considering doing this, since I may send mine back to the factory for other service as well, but I haven't decided yet - it's fairly expensive. (Oh, and make sure to have your B+ fuses and holders uprated to the current-production 600v type while you're at it; the old 250v type is not adequate protection.)