I have a Ashdown ABM-500 amp that has a built in tube pre-amp. Can I use a external pre-amp with good results? I am sure I wouldn't do any harm but would I hear "warmer" results. I know the mix knob on the Ashdown say's "grind" but I would like to try "warm". Would this be possible by changing the tube in the Ashdown? Thanks for anybody's time.