When buying a K 340, ask the seller about any channel imbalance in the mids (2-4 kHz). I've seen measurements by Tyll of two pairs of K 340 and they both had the imbalance there. My pair did as well. You'll want as little of it as possible. You probably also want to ask about an imbalance in the treble, which you want none of.
If one was going to drive the K 340 out of a weaker amp, I wouldn't recommend paying more than $70 for them, and even that is probably a bit high. They sound neat, but also have problems (channel imbalance and roller coaster frequency response for starters).