If you can get past the sarcastic posts by people who should know better, there are some very pronounced differences between battery types. An alkaline battery will have a 1.5 volt cell, while a rechargeable will be about 1.2 volts. However (and this is a big however), alkalines will show a steady voltage drop over the life of the charge, while rechargeables tend to hold a steady voltage until the charge life is almost done. So, if your amp needs maximal voltage, you'd probably want to go with alkalines, but the sound would deteriorate long before the charge was gone. If your amp can run on the lower voltage from rechargeables, then you'd have the advantage of the slower voltage drop over time. However, the charge time on an NiMH battery will be less than an alkaline. Even so, the advantage of rechargeables is that you can, well, recharge them. So, a set of rechargeables will last a lot longer over time than a set of alkalines. Rechargeable batteries will lose their charge sitting, so it's best to charge them as close as possible to when you use them. I generally keep a set in an amp I'm using, with a spare set ready to go in when the charge wears down.
Oh yeah, a suggestion dedicated to bigshot and Uncle Erik: Do treat the terminal connections where your battery hooks up with Quicksilver AVExtreme. This is a wonderful contact enhancer, and will help insure that you don't have a poor connection between your batteries and your amp. It tends to be a bit expensive, and probably not worth it on a portable, however, if you've ever had connection issues, such as an intermittent battery connection, or poor connection with tube pins, RCA connectors, etc. this stuff does work very well indeed. Be careful using it, as it is highly conductive and if you paint a short it can do some damage.
