Crisp highs and tight bass are not well defined.
Obviously you need enough power to reach your desired SPL. The cheapest options I know of for decent power are pro amps and Emotiva amps.
I won't touch the issue of subjective sound. I personally think amps are going to sound a lot alike. Some might have less distortion, but they way they rate amps is not going to tell you what you want to know (THD is not some absolute measurement for example.)
And of course, a power sub will likely help with low bass (depending on what you listen to it might help a lot.) I bought the Emotiva Ultra 10, which is a great budget sub so far.