I have a Mark IV here and have owned II and III.
It can get similar amounts of gain and saturation with the lead channels. The Mesa might be more saturated. The Mesa amps are more compressed and have a midrange heavy sound (without the GEQ).
The Lead channel of the Sig X can get similar sounds. Each has it's own individual flavor. The Sig X is going to be more dynamic, articulate, better definition of notes in chords. The Mesa is going to be easier to play and has that unique midrange heavy Mark flavor. The Fryette has a more balanced EQ.
Both amps are capable of a wide variety of sounds.
The entire range of the Fryette tone knobs are usable. As you know the bass knob on the Mark you only use a limited range.
The Mesa amps have more bass available. Very much so with the GEQ.
I much prefer the Rhythm channel of the Sig X over the Rhythm channel of the IV and III. It is the channel I use the most on the Sig X. It does great at getting vintage English crunch sounds.
The clean channels are very different.... The Sig X Clean channel is much more like a vintage NMV amp.
In general the Mesa is more like a Fender and the Fryette is more like a Hiwatt. I sometimes describe the Fryette amps as a combination of Mesa Mark and Hiwatt.
If you have a more direct question about details or just other questions I am very fimilar with both amps. I've had the Sig X since it came out in 2008. I've always had a Mesa Mark since 1982.
I am curious what modifications have been done to your Mark IV ?