Well it sounds like your on the right track if your buddies are digging your tracks.
It sounds like your happy with the recorded tone so it's probably an EQ / Mixing thing.
Balancing the bass with the kick can be tricky. If your going for a big kick sound, your not going to be able to have a huge bass sound as well, or you'll have a big boomy mess. In this case, focus on the growl of the bass and thin the really low end so you can turn the bass up in the mix without it booming on ya. The UAD Pultec EQ is killer for shaping the low end of kicks and bass guitar, and can punch up the mids nicely too on a bass. Conversely, if you want a big thick bass tone, your gonna have to carve out space for it from the kick sound.
Also make sure your high-passing filtering just about everything else in the mix to keep the low end of the mix nice and clean. There's nothing much usable below 100hz on most tracks.
If you do want to mess with the recorded tone, have you tried skipping the POD and plugging straight into the instrument input of the LA-610 or directly into the Apollo hiZ input?? Unless your coming out of the POD digitally, your adding an extra round trip AD/DA conversion before you hit the Apollo AD. The simpler and cleaner the signal chain is, the better. Too much crap in the signal chain can cause the low end of the bass to smear and loose punch.
Another thing you could try is your bass straight into the Apollo Hi Z input with the API Vision console Unison plug-in running. You could EQ and compress your bass in real time and record the resulting tones. API EQ is killer and their compression is super punchy. You can demo that for free for 14 days if you don't already own it.
If you do try the Sans Amp (which is actually a pretty cool device), plug the output of the Sans Amp directly into the line input of the Apollo.
Or, if you want to track with compression, Sans Amp into the line input on the LA-610. I'm just not sure what the Sans Amp is going to give you vs. what you are getting from the POD.
Hope that helps