I'm not sure what you mean by "through a keyboard amp". The DP004 has some reviews written about it, in one I found this segment:
"Sound Quality/Microphones
The internal mics of the Tascam DP-004 sound very good. Warm, natural and fairly quiet. Unfortunately, the signal we achieved using
the Audio Technica AT822 as an external mic was not quite as strong or clean as the one we achieved with the internal mics.
The Tascam DP-004 does not have phantom power (48V) or plug-in power (~5V), so your microphone choices will be limited to the internal mics, battery-powered mics, or a condenser mic with an external phantom power supply.
Based on the examples in the Owner's Manual, the DP-004 may sound best when using its built-in mics for vocals and the 1/4" inputs for guitar or other instruments. "
http://www.wingfieldaudio.com/tascam-dp-004-review.html
anyhoo...
A SM58 is a good vocal mic, but it is ideal for up close singing like a live performance. It can still work fine for recording, but it doesn't behave the same way as a condenser. I'm not sure why you are going "through" your keyboard amp (or even what is meant by that); Are you using the internal mics to mic the keyboard amp you are singing the SM58 into? Confusing. If that is the case, you are just adding an extra step where the internal mics could catch your voice just fine instead.
Also a lot of vocal tracks can sound really dry by themselves, even if it's a good singer, the microphone may emphasise certain frequencies over others, catching a lot more 'bass' if you sing close to it. A touch of EQ can fix that by pulling the bass out a bit, and a tiny touch of reverb to make it sparkley. I can't remember if the 004 has an EQ and reverb I know the big brother
DP008 does, but if you can find a way to get a dry track into it, and then use some minor tweaking instead of just the dry sound of the mic, you might like how it sounds better than what you have got going on now. Just a suggestion.