I am generally not a fan of modifying new or current era equipment,
whether audio, musical instrument, or T&M equipment. You lose the
warranty, resale value, and whatever "special stuff" the box had or
has. If you want a new or used-with-warranty or used-factory-supported
piece, buy one that does what you want in the first place. If you want
to mod/tinker/experiment start with something fully depreciated and
with no collector value (there still are a few tube audio pieces in
this category, roughly, if you are willing to scrounge-eBay has no
bargains there) or build from scratch, or salvage someone else's bodged
job.

Cost of ownership on many very decent used high end audio pieces can
be very reasonable, inasmuch as if you buy them right you can sell them
for what you paid for them.

Mods from recognized shops with recognized expertise can be a
different story, the key is, do they really know what they are doing?
Some do. Many DO NOT. Keep in mind some manufacturers can be real
hard-ons about dealing with modified boxes, there are modders out there
who cannot do basic troubleshooting and repair and if they get hold of
the box and it fails or you do something dumb, it may wind up in limbo:
the modder can't fix it, the manufacturer won't let it back in his
door. And if you live in a city that's not a major city there may not
be any competent repair shops in town anymore!

If you want to learn this stuff, you can, but don't think you will get
off without a thorough knowledge of basic electronics and some 'craft
skills'. The place to learn them is on junk, not expensive pieces.