508 18 bit upgrade or just add a dac, benchmark?

Just got a Meridian 508 18 bit. I got it as an upgrade from my vernerable NAD 540. I was hoping that the Meridian would be a warmer, fuller, richer sound with more bass. Although it is a bit warmer than the NAD it is still not as full and warm as my uncle's Meridian 506 16 bit.

My system is a bit on the bright side (I think it may have more to do with the room acoustics than the components...hardwood floors). Some of my favorite punk rock sounds a little compressed without much seperation. While the Meridian is a little more relaxed than the NAD It needs to be more smooth to make long listening enjoyable. And the bass doesn't seem to be much better than the NAD...I thought Meridian were supposed to have great bass?

My system is the 508 cd player, Musical Fidelity 300A 150watt integrated amp, and Vienna Acoustics 2way Bachs speakers. The speaker wires are the audioquest CV-4 and the interconnects are the audioquest Corals.

I guess my question is, do I go for the Meridian 24 bit DAC upgrade (still $750 through Meridian) or do I go with an outboard DAC, possibly the new Benchmark or Meridian 566. Maybe I chuck the Meridian all together and get something completely different.

Any bit of input would be great...I just want to be able to listen to my music for more than an hour without being irritated.

-thanks, sean