Upgrading amplifiera.

Without picking on anyone in particular, there are some wayward responses above. The correct answer is that it all depends. The amp has to mate with the loudspeaker. A 16 ohm ultra sensitive horn doesn't need much more than a watt or three and a planar may need a utility sub-station. Pass Labs covers the gamut more than any other top manufacturer in that regard. Beyond a doubt, some enthusiasts become fixated on amps as the focal point of their system just as others focus on their ever-changing search for the "perfect" loudspeaker, etc. Personally, if a speaker is hard to drive and has contorted impedance curves and phase shifts, I wonder why bother. The analogy is to the old perhaps outdated scenario with MS vs. Apple-MS operating systems were designed with the idea that if you threw enough computing speed at them they would work fine and Apple operating systems were designed to be less taxing. Unlike the three major OS's (Linux, Apple, and MS), there nearly unlimited choices as to loudspeakers. 

Some manufactures are very very reasonable for upgrades. I was a dealer for Bricasti and Brian Zolner takes excellent care of his customers. Talking in the $150 to $300 range. I remember on guy who was upgrading the Antique Sound Labs Hurricanes. He charged more then the retail price of the amp. 
Seems silly to spend almost as much on the amp as it is worth.

We have an update with warranty reactivation policy. We also offer options- higher performance capacitors and resistors. We've had these options for decades, they are easy enough to hear.

We prefer to do the work on the amps so that we know its done properly and so can stand behind it if there's a problem. One customer sent us a preamp that had been 'updated'... whoever did it left a rat's nest of flying wires and parts inside the preamp, causing us to have to write an estimate to repair the preamp.

If you are having someone do update work, it is in your best interest to google as much as you can about them so that you know they are doing good work. Some, like Jim McShane who does excellent work on Citation 2 amplifiers, are easy to sort out and others might seem pretty obscure and might be cause for concern.

BTW, a few hints: hot melt glue to hold parts in place is not acceptable! Terminal strips cost a few cents; there is no excuse for parts flopping around inside! The updated work should look like it was born there.