[quote name='fonzoooroo' post='1167951' date='Mar 19 2011, 08:35 AM']Welcome to the minefield!
Fom my experience:
Strings and technique dictate action height. Players using a soft string (weedwhackers/guts) use a very high action to get" underneath" as you describe... but if you go for a higher tension string, you won't be able to slap on a very high action.
I currently have 2 electric uprights, one with spirocores (for the mag pickup) and a very low action (gives a psychobilly sound, for want of a better description! - perfect with a loud band though.), the other with some sort of innovation nylon cores on (peizo pickup - much more "natural" sound) ... Both have 4/4 strings on a 3/4 scale length and a correspondingly higher action for the one with nyons (even then, it's only regular "classical" action though!) and I play slap on both.
I'd assume you're after more of a "thwack" slap sound rather than a "click" ? if so, that's going to be gut strings/whackers or nylon core (possibly bumped or solo set de-tuned) and a high action.
There's lots of chat/reviews/rants on here about amps... have a read of those, and you'll quickly get a shortlist together! ...It all comes down to power (how loud your drummer is) in the end though! A customer of mine just bought a 700W GK beast for his DB and is very happy. ...
I run a ridiculous mixer/POD XT/Behringer shark (just to pre-amp the clicky) /PA power amp/big peavey 2x12" PA cab... but that's far from normal!
I'd offer lutherie, but I'm a bit of a hack down the country for you![/quote]