distorted low end and phasing

hello everybody!!
I just recently purchased and ampeg svt II-p
preamp and an ampeg svt 18e cab with a crate spa-200 power amp. my problem is that i get a great sound in the register of the a-d-g strings but as soon as I hit the e string my amp starts to distort the notes. now this happens both on my upright and my electric basses. I realise through the preamp (since it is a little noisy) that there may be a grounding issue. Also there is a weird phasing going on, slight, but there. If there is anyone with any suggestions on helping me find a nice full, rich, and powerful bass tone through this rig, please help. Thank you so much. Peace.
--Pete