I knew immediately when I saw this thread title that it would be the Ironball vs. one of the Mesa mini models, and I knew immediately what my answer would be. I have played them all and here's the way I'd break it down:
Overall tone - Ironball wins
Versatility - Mark V25 wins
If you want the Mesa Rectifier sound at manageable volumes - obviously...
If you want the rectifier sound with reverb - obvious.
The Mark V25 vs. the Mini Rect or Rectoverb isn't a matter of which one sounds better. Both of them sound just as good. It really depends on what you're going after. The rectifiers have a unique sound, and the Mark V has the Mark IIC+ and IV circuits. All of the rest of the gadgetry of the Mark V 25, like the 5-band EQ, is icing on the cake.
The Ironball is just a really, really great sounding amp. For an EL34 amp, it has a decent low end punch and it's not ice-picky as the Mesa stuff can be. Oh yeah, the icepick and all of the common Mesa Boogie qualities are in the mini versions (although I've never really been bothered by the so-called ice-pick, but it can sound spiky if you let it get away from you). Overall, I think the Ironball just sounds fuller and has more articulation in high gain. I also like the Ironballs clean channel more than the Mesa Boogies even though the Mesa stuff gets a little janglier and, I suppose, more in the Fender ballpark.
They all have great FX loops, but the Mesas may have a slight edge in FX loops as they do just as well with digital or analog effets. The Ironball seems to sound better with analog effects. Keep in mind that my only tests were with my analog pedal board and a Line6 M9. In clean or pushed clean of the Mesas, where you want to hit it with an overdrive or boost, it develops this compression that subdues the sound of the low EAD strings and is more pronounced in neck position. I think the front-end of the Ironball is a bit more pedal friendly.
The Ironball cuts through a mix probably the best because of the typical Engl low midrange, etc... The Mark V cuts through better than the Rects. Overall, the Mesas are a bit louder I think, but the Ironball isn't slacking with 20 watts. They're all gig-worthy for us mortals who probably only play smaller venues.
Ironically, as far as speakers are concerned, my Mesa Black Shadow speaker sounded better with the Ironball than the Mesas. I particularly liked how the Mesas sounded with my Mesa 1x12 loaded with an Eminence Man'o'War (I have 3 Mesa cabs).
When it comes down to it, there's not obvious choice. Mesa Boogie has been outstanding in the way of providing us with a fantastic variety, and Engl has put out the ultimate low-powered Engl that compromises on nothing. Personally, I prefer a bit simpler amp with less fuss, so the Ironball is my personal choice.
I hope this helps your decision man.
