Well, first of all remember it's a motorcycle, not a concert hall. Second, send your radio to LarryM here on the board. He will upgrade your radio and probably remove the need for an additional amp. Third the db501's are a good choice.
Honestly looking back, I almost wish I had NOT done any stereo upgrades. Reason being is I very seldom listen to music via external speakers. mostly and especially when riding always through the headset.
Comparatively speaking, when riding with some of the Harley boys, their stereo sounds certainly blows mine away even after the upgrade. But most of them dont have headsets either. To enjoy the music over and through wind noise while actually riding. Just monday morning feedback. But still I like the way my systems externally sounds