I'm actually running my Alesis DEQ-230 digital 31 bander in my effects rack which is in the loop of my Mark III or V. All the good things you can do with the 5 band EQ to shape your tone can be improved on substantially by putting a 31 bander in the loop. Yes this is SO true.
An interesting way to do it is to start by setting the 31 band EQ flat and use the amp's own 5 bander. Then the 31 bander is used only to make small tweaks to your 5 bander's curve. This way you START with the crushing tone you love, and only refine it, rather than having to rebuild it completely from scratch. Which might be a bit confusing if you go by the EQ freqencies printed on the panel of the Mark amp, because they're NOT accurate to the real center frequencies of the 5 filters. By using the 31 bander only as a trimmer EQ, you avoid that confusion.
The Alesis is a nice unit for many reasons, but best of all, it has 30 user programmable memories so you can set up 30 full 31 band EQ settings in stereo. Or treat it as 60 if you address the EQ as two mono units instead of one stereo unit. The EQs can be decoupled, with a separate curve in each channel, or they can be linked which is the default setting.