best used amps < $500, < $1,000, < $2000?? or better to buy new??

What are the best amps/pre-amps to purchase/own < $500, < $1,000, < $2000??  I have different pairs of loudspeakers: IMF RSPM Mk. IVs (3 sets), B&W, KEF 4-driver sets, ADS L9?? & others. I am trying to really become knowledgeable about what amp/pre-amps offer the best bang for the buck in sonic quality, build and long-term enjoyment. Is it Hafler, Rotel, NAD, McIntosh (not sure if this fits my $2,000 or less criteria) and the multitude of other amp only, amp-pre-amp choices. Are there any  amp only, amp-pre-amp options from manufacturers whose wonderful engineering & quality just don't get any respect & fly below the radar for way less than more exalted mfgs. Thanks very much for the mentoring & education in advance. Or is it the case that a new NAD or other is head & shoulders above any vintage, used amp, amp-pre-amp combo??? Much appreciated, TLL