I went for a RAT clone in the end; a Mooer Black Secret. I've found that I prefer the 'Turbo' mode and I keep the gain below about 10 o'clock. I use it when we play Black Keys 'Lonely Boy' and will probably use it for some 70s punk songs we plan to add to the set list. Sounds different enough to the SL drive and the dirty channel on my Marshall, so worth getting for £32.
I've also replaced my EHX Soul Food with an MXR Timmy. I found the SF sounded a bit too nasal, but the Timmy is adding just a nice low amount of grit on top of my Marshall (which is set just on the beginning of crunch with the gain at about 2 o'clock on the clean channel).
I'm late to the party in terms of stacking, but I found that the Timmy into my Xotic SL drive works well (and I can use either one alone for various songs). I also like my Xotic SP compressor into the Timmy as well: This sound is more like using the Timmy with the switch to the left, but it means I can use the middle switch position, which is more 'open' sounding, then kick the SP in if I want a more compressed sound. It's easier than bending down to fiddle with small switches mid set.