It's not a good idea. The last thing you want on a night ride is to short your headlight fuse because you over drew the circuit listening to music loud.
If it s a decent amp, it has a remote lead/terminal. The run from the front of the bike to the battery is not "long" by any stretch of the imagination. If you want to know long, try working on an Escalade. Run the
main power wire and remote wire back to the battery. Tap into a fuse in the fuse box for the remote wire (use a light tester to tell you which is the actual "hot" when the key is on) any switched wire in the fuse box will do for that, and hook up the main FUSED LEAD for the remote to the battery positive. Ground out to the bike frame as close to the amp as possible (which is probably the battery negative, but if there's a place to ground out sooner, you'll reduce potential for interference), with the heaviest gauge wire possible.
Seriously, don't overload critical electrical components. On a bike, that's all of them.
Even if you use a relay, you're going to need to run, minimally, a positive lead from the battery positive to the relay to the amp. Relays are not necessary if there's a remote lead on the amp. If your amp drains the battery, one of 2 things is true: your amp sucks OR, you didn't hook it up to shut down when you turn off the bike, and, well, of course it's going to drain the battery.
Have I ever mentioned on top of doing detailing, I spent about 10 years installing mobile audio professionally, and a better part of my life doing it casually? Heh.
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ya have to have key onto run the amp, I can't see it taking more power away from the charging system then say highway lights? 
if ya have head light off the center wire that runs to your lamp is the regular light, not the high beam, and come on with the turn of your key. I do that one just me if ya don't want to cut wire try taking lamp plug out and striping wire for amp you should be able to plug it in to lamp with lamb wire to see if is what you want it to be good luck.
RELAY SUCK
I still unplug it if it going to sit along time
a switch to turn on and off the amp after starting bike that the ticket let us know what ya did.What I said earlier was to test it to see if could draw the power that way. Then put switch on it
was in my head just didn't type it down that way good luck
you will be answering a lot more questions now. thanks good to know.
