I have a four mod SLO, depth, WH, 1/2 power and scoop. It was priced well otherwise I probably would've picked up a stock one. I've gigged it a couple times a week for 7 years.
I've never used the Warren Haynes mod, it's essentially a bright cap "clip" on a switch, I don't care for it and I don't think too many people do. It has a dulling effect I don't think most would like.
The 1/2 power switch seems to change the attack slightly (maybe a little softer). Not sure why this is offered, if you're looking for power tube overdrive, this is probably not the right amp for you in the first place! I had tried it early on but haven't for a very long time. Some may like the slight variation in feel/attack.
The scoop mod, as mentioned above, gives the normal channel a Fender vibe. Seems like a cut around 400hz. It has it's place but I don't care for it in conjunction with channel switching with the overdrive channel, just my preference.
I saved the depth for last as it's the most polarizing ;-) ...seems to be a love/hate thing. I rarely used it in the past but the last couple years have been experimenting with it. I didn't use it before because I typically don't like a ton of low end. Lately I have been running the bass control down around 3-4ish and running the depth up around 9 o' clock. It changes the voicing slightly in a way that I've been liking. Soldano purists will bitch about it I'm sure, but it's worth a try. It tightens the bottom up a bit and the attack feels a little quicker or snappier.
...and as usual, YMMV!