My current bass rig is a Hartke 3500 head hooked to a Hartke 2200 cab. The rig sounds simply amazing, especially with the Cirrus plugged in, but I am not too dazzled with the tube pre-amp sound. It seems from what I have read that others also have had this issue. So, what tubes do you recommend as replacements for the factory one in the 3500 head? How hard is it to replace? Costs in volved? Can I do this myself (I have an engineering background and work on computers/networks daily)? Thanks!