Why do you want a 4100, what are you trying to accomplish?
I have a Macintosh mc402 amp, mainly because it pairs extraordinarily well with my speakers. It took some time and experimenting (and $$$$) to figure that out. Every other piece of equipment I have is a different brand, and has been modified heavily, because that's what made that part of the chain optimized for the next part of the chain.
So to get started, start buying equipment and scientifically evaluating it against something else. If you can't afford that, see if you can find some local groups to get together... Go hear something you can experience in person.
McIntosh holds its value so if you do buy one you can get out of it without losing money usually. I think of it as a good investment, but ironically it's the last amp I bought 10 plus years ago. I could sell it today for at least what I paid for it, but I don't know of anything that I could replace it with that would be significantly better at a similar price... In my system, to my ears, etc.
