I'm needing a small inexpensive tube amp and I was wondering if there is any difference between the older Pro Juniors and the newer ones? I've heard that the older ones used to come from the 'custom shop' factory and now they are part of the 'hot rod' series. Are there any major differences in components or build quality? Are there any good aftermarket mods for these amps besides changing out the tubes and speaker? Thanks, Perry D.