Gary, your needs are simple. Don't get all worked up over this. You have several inexpensive, lightweight choices, and ENDLESS expensive, complicated ones! First:
1) If stereo is not that important - you can use any brand of keyboard amp on the market today. Some nice choices are Yorkville, Peavey, Samson, Fender . . . get the idea?
2) If you require stereo to keep you happy - you can buy two small amps and plug in directly with no mixer needed. The only bad side to this system is - you have to make two sets of tone and volume settings to get a good balance. In the REAL world - it makes little difference. Plug it in and turn it up till it sounds good. Then don't mess with it!
3) If "Really tiny" speakers are attractive to you - there are a number of decent systems made for computer sound by Roland, Yamaha, Fostex. They often come with a sub-woofer and two satilite top end speakers. All powered. Just plug in a stereo cord and you're set to go.
4) Buy an amp with a horn for the high frequency. This will make your piano and string sounds come to life. A "guitar" amp will not reproduce all the frequency range that a modern synth can generate. If you have a strict budget and size requirements - tell me and I'll be more specific for you.

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Uncle Dave
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