I say forget the Roland, why because all Roland digital recorders like the one you mentioned, uses *data compression*. Data compression compresses the audio into a smaller file so you can fit more audio minutes on you hard disk. There's one catch to this; data compression screws up your sound. I assume you are looking into digital recorders to put out a clear and crisp sound right, well if that's true then data compression can be a your worst nightmere.
I recommend the Tascam 788 that you mentioned, but even better than that, a Korg. I don't know model #'s, but any Korg stand along digital recorder is great, not just for the money but for the quality.
You definitly need a mic preamp. I say either the PreSonus Blue Tube (2 channel, $160), or the Art MP (1 channel, $100). The PreSonus gives you a better value for your money.
You can buy a mic cable that has an XLR cable and on the other side it has a 1/4 cable, so you don't even need a $15 adapter.
A great Bass DI is the SansAmp Bass Driver DI. It emulates a tube bass rig for $190. I use this sucker and sounds damn good. Another Bass DI emulator is the Line 6 POD BASS. It has cabanet emulations, 16 effects (even a compressor), and an EQ for $350. Keep in mind the SansAmp only has one emulation, but sounds damn good, the presence knob is killer! I don't know how THe POD sounds, I never tried that stinking thing.