I have no experiance with any of those amps, but based on my experiance of JBL with car audio (thought this was a few years back), i really cant recommend them. The sound was clear but very sharp treble and a bit slow on the bass side. But that said, i heard the JBL Creature 2.1 PC speakers a few weeks back and they and a nice tonal clarity to them and a bargin for the sub-£100 mark IMO.
With the Marantz, i was looking at that myself, but the review of the 6003 was much better than the 5003 (5 star vs 3 star), worth the extra few ££.
I'm no expert either but ive put up quite a few posts over the last few days about various amps in this price bracket. What type of sound are you looking for? For what type of music?