There are a couple things to know about transformers and if you will reap the benefits from a premium set.
The PT is like the heart and circulatory system. Think of it pumping current like blood through the amp. Now consider the demands you put on the amp. Bedroom levels are like a slow walk with your dog. Cranking it is like running a marathon. At low levels, you are asking very little of the PT. So an upgrade makes little financial sense if you aren't really using it's potential. However, if you crank the amp up it demands a lot more of the power supply just like a marathon does of your heart. That's where a quality PT outshines every time.
On the other hand, your OT is like the lungs and voice box. It pairs the signal leaving your output tubes to the speaker, operating like a fulcrum if you will. A high quality OT will also shine regardless of the level you use it at. From a designers POV there are considerations such as primary impedance, laminate technique, oversized vs undersized and more. I typically offer two custom sets for the RM-series amps. One is tuned in the sweet spot for the most tubes and another that makes EL34/6550 tubes more at home. When you claim the ability to use any octal tube with an amp, some types of tubes are going to operate happier than others because you need to make choices and not all can be ideal for all tubes.
So in short, you can see an improvement with an OT upgrade nearly always (in addition to going with a design that's best suited to your favorite tube) and a PT upgrade is mainly judged by your application. This is part of the process I guide you through when selling an upgrade job.
The RM20/22 iron is underwhelming in stock form. Upgrading these and adding a choke is pretty night and day over stock form. That's also part of the secret sauce in the amps I build for our showroom here at the shop. I've been building custom amps in the style of a Princeton Reverb that I can't keep in stock. They sell as fast as I can make them. I redesigned the power supply and use an oversized Deluxe Reverb style OT. Along with some other things, this amp yields 5W RMS more than any Fender and with a much bigger sound from the same small package. It's a Princeton with enough horsepower to hang with drummers. It's similar to the RM20/22 upgrade.
I'll close with a story that helps explain it in context. When I did George Lynch's RM100LB, I delivered it in person at a tech gig I was hired to do for Lynch Mob. Oni (Logan, the vocalist for Lynch Mob) has extreme sensitivity to loud guitar amps from years of gigging. When we set up the stage, George's amps need to point off-axis about 45 degrees away from from facing the audience and in the opposite direction of center-stage. We were using an SLO and a Trainwreck for George at this show and tried the RM100LB I did right after soundcheck. Oni came up to George and I and complimented how good the amp sounded at high volumes. He found it very comfortable at high SPL without any painful artifacts that he experiences with most amps cranked that loud. It's an interesting story of what the upgrades can do in a real world setting. That amp had the EL34 transformers I offer with a quad of EL34's.
Hope this helps shed some practical understanding of the what and why of transformer upgrades.
Rob
