Line 6's strength and bread and butter is their modelling. Like Behringer, I'm skeptical of anything they put out that has a power amp in it. Their bass stuff just seems to be there because it seems they are supposed to do bass stuff too. I don't think their bass amps have been around long enough to know yet if they're any good. Of the 11 thumbnails on Line 6's main page, bass and vocals are the smallest ones. That may be an indication of the level of attention they get. I was considering the LowDown 750 a while back. I never got to play one, but the modelling appealed to me just for the simplicity of EQing. I love the "scooped mids" sound in my living room and at low volumes, but playing in a room with a band it's hard to keep that from sounding muddy. I'm not crazy about the boosted mids sound but use that most of the time because it can cut through a mix and any articulation isn't lost. "Line6man" here on the forum is a bass player, and quite a tech saavy one at that. If he chimes in and his Line6 moniker is bass related, he could certainly give the best imput.
Pros (IMO)
-modelling
-1/4" line out (dry)
-XLR line out (wet)
-built in tuner
-built in compressor
Cons (IMO)
-modelling
-no tuner out if using a rackmount jobbie (1/4" direct out could work as one)
-no level control for either line out signal
-no effects loop (since it's a modeller they assume you don't need or want one I guess)
-no sweepable mids
-other than combos, they make 2 heads and one cabinet for bass
-their heads' watts mismatch their cabinets at properly matched impedance levels. To get 4 ohms of Line6 cabinet, it takes (2) 8 ohm 600 watt cabinets, so 400 or 750 watts of head for 1200 watts of cabinet?
-a quick search yields negative quality/durability issues but excellent warranty and return policies
-possibly made China - not automatically a redflag as most stuff is nowadays, but just throwing that out there