I'm not an "expert" but my first amp upgrade was a used Adcom GFA 545 from Audiogon. Still works, and I still use it! I'd keep an eye out for them used, watch the wattage on some of them as they can be low.
The 545 for $200 was slightly nicer than the amps on my H/K AVR8000.
I've since upgraded to (used) Parasound Halos, but that's not what anyone would consider "inexpensive." Still, I don't think I'll need to upgrade for a long, long time.
I need to do a comparo against the Halos and some "pro amps" I have for the subs sometime. I'd LOVE to save a ton of money, but I've already heard differences among amps.