I do not have any problem purchasing equipment upgraded by the manufacturer. An Audio Research REF 250 amp upgraded by Audio Research to a REF 250 SE means that not only did Audio Research upgrade the device, the changed the faceplate and whatever they needed to do to make it basically a REF 250 SE. they also service checked the amp to make sure it operates within their specifications and standards. Who better to check this? It is now a REF 250 SE amp.
Maybe the sale price would be slightly lower than an original REF 250 SE amp.
equipment upgraded by anyone other than the manufacturer would require some more investigation on my part.
I for one do not purchase with resale in mind. I purchase based on my musical/audio needs at the time.
I upgrade and modify equipment often. for example, there are much better more linear transistors available now than in the past. So if the transistors have the same specifications, but are much more linear, that is a natural replacement. On my Bedini 250/250 MKII amp, I have upgraded/modified by replacing all the output transistors with much better more linear transistors and also the drivers. I now have separate power supplies for each channel with much more power supply capacitance and bypass capacitors also. I installed a slow start circuit, installed Threshold like monster heat sinks, removed the speaker fuses, used Cardas internal wiring, new/better power cord, individual rail fuse protection.
I spoke with John Bedini while I was doing this (before he passed) and he liked my ideas. He even searched through his storage facility and found some unstuffed circuit boards that I could use. I increased the bias and this amp really sings.
Is it original? nope. If I were to sell it, I would make sure the buyer knew about the upgrades/modifications. It is my backup amp to my two Audio Research REF 250 amps and my Mark Levinson 23.5 amp.
I've done similar work with Robertson Amps and many Threshold amps.
enjoy