Not super powerful but way more professional than a boombox - I think that would be pretty cheesy.
I've always found humor in the suggestion that "professional looking" magician equipment is somehow superior. Function is function - cost is cost - if the product does the job, who gives a crap?
Again, if your spectators are focussing on your sound system and how "unprofessional" it looks - you need to work on your act.
If your spectators are focusing on your sound system it's because they can't hear you or it sounds so distorted that it is distracting.
You simply can't fill a large room with a boombox. It's about the audience's ability to clearly hear you in a larger venue; from the front of the room to the back.
PA's are designed for sound reinforcement and boomboxes are not. Boomboxes do not have the power to drive large speakers, there is no mixer on a boombox, they do not have the proper microphone inputs (XLR and 1/4"), probably no RCA inputs, they do not have separate channels to control individual volume levels and EQ settings and no master volume to control the volume of the overall mix.
Real PAs for small to mid-sized rooms are not that expensive. You'll need more power and larger speaker cabinets for doing larger rooms, auditoriums and outdoor settings. If you are doing professional gigs on a regular basis you will want to invest in a real PA. Use the right tool for the job and you'll sound (and look) professional.
Musicians Friend has several inexpensive PAs (under $300) and wireless mics.http://www.musiciansfriend.com/navigati ... 7c0&page=1
I believe I covered this in a post on the first page.
If you are performing at a location that requires something larger than a boombox, chances are they already have a sound system in place. On the rare occasion that they don't, you can just rent one for the day.
If the sound is so bad that they can't hear you, then yes it's a problem; however, the matter you're describing has been addressed already. YOU do not need to buy the amp, because there will likely be one when you get there.
Back to "looking professional": You will not "look" any more professional if you have a better amp. People will hear you better, that's all. I continue to see alot of humor in the "looks professional" premise - especially since 90% of the times I hear it used it refers to some irrelevant aspect.