IMHO if you have a Mark IV and a halfway decent Bass any issues you may be having with your recorded tone have nothing to do with not having an outboard eq. Try investing in a different or additional mic instead, ie: Royer 121. Great tone originates from the source 99% of the time and your Mesa, when tweaked is more than capable of getting you there. I track with an older Mark IV quite often running EL's and 6L6's through a Royer 121 and an API 512c and never feel the need to pop in a 550b or anything else for that matter. Best of luck