Is it caused by the transport or the dac?

I am auditioning a new transport. I am currently using a Theta Data Basic II with the Musical Fidelity Tri-Vista 21 DAC. I'm looking for a transport because the Data Basic II is rather old, I had to replace the drawer unit last summer, and basically I'm concerned that's its time is near.

Auditioning the new transport is puzzling. There's what seems to be 'funny' things going on, and I don't know if it's in the transport's reading of the CD, or the DAC's receiving and translating what's being sent to it.

At times the bass is almost non-existent with the new transport and at other times fine, whereas the Theta Data Basic II is consistantly powerful in the bass region. The bass seems to disappear more when the music gets complicated.

The same for the midrange. If the music is simple, i.e. vocalist, one or two non-distorted guitars, bass and simple drums. the sound is much better than through the Theta. Better textures, dynamics, etc. However, add distorted guitars, overdubbed guitars, powerful drums, basically anything more complex, and a general smearing occurs where the clarity of the individual instruments and vocals had been.

Sometimes there's even a very quick stop in the music throughout the song, like 'I can't figure out what the data is that I'm dealing with, so I won't output anything!'

This only occurs with the new unit, it doesn't happen at all with the Theta. Like I said though, on simple songs the music sounds quite a bit better in all areas.

Is the transport having the problem reading the data in the complicated passages? Is the transport sending more information to the M.F. Tri-Vista 21 DAC than the DAC can process? This is perplexing!

Thanks,
Chuck