To be considerate, I believe not all the Mark V amps will exhibit the brittle tone that becomes apparent using 90W full power at gig level. I had issues running the Mesa 6L6 STR440 with my amp early on as I kept red plating them. I had to move to the SED =c= 6L6 GC as I found those tubes were capable of surviving the higher plate voltage and what ever was going on with my amp in general. Note the SED =C= 6L6GC tubes are not as forgiving to the ear as the Mesa STR440 6L6 tubes due to their difference in frequency response. In general, most 6L6GC variants will have a brighter tone when compared to the Ruby 6L6GCMSTR which is what Mesa is rebranding after they sort out the good tubes from the not so good tubes. You may not even need to change anything. However if you find your amp getting into that ear piercing brittle region at gig level and if you find you need to back off the gain to 10 am, treble and presence to nil just to have a pleasing tone that may be lacking character (sterile) then the V4 saturation mod will help considerably.
When I went to try out the New TC-50 in December I made sure to try a new Mark V combo to compare to what I have now. That amp sounded great!. Not sure the production dates but there were some issues with components failing. Screen resistors on the power tubes and a few that had a bad batch of 1N4007 rectifier diodes. My amp was made in 2012. So far after the repairs the amp has served me well but did not correct the brittle tone at elevated levels. I could not use any V30 speakers with it as that made it far worse. (EV was no better either as those definitely will dump the top end out with impunity and make those in the room suffer the consequences. Ouch). The cure with the brutal breaking glass tone with some Mark V amps is a tube roll on the preamp circuit or as of late the revelation of the saturation mod, all stock tubes and change V4 from the Mesa 12AX7 to a JAN/Phillips 12AT7 or similar. The amp will still be bright but not brutally brittle. There is still plenty of top end to cut though any mix. The 12AT7 alters the 4th cascaded gain stage that boosts the midrange frequencies and increases roll off of high order harmonics (contributes to the brittle tone). However, in most cases you can use other preamp tubes to achieve a similar effect but will not enhance the lower midrange but overall will be satisfying. Say you had interests in the JP-2C, there is one sure way to get that tone and more with the swap of V4 to the 12AT7. I have both and it is awesome that the V can emulate the JP characteristic with the simple mod. No soldering required and you can easily go back to original tone (after you let the tubes cool down to gain access). I was ready to sell my Mark V and after I tried a new one, I would have replaced it with a new V eventually. I love the amp that much it was deserving of a second try. I am lazy and did not want to take a bath one selling my V, the saturation mode saved the day such that it rendered the love/hate relationship to just love. The saturation mod could easily be called the Mojo fix. Definitely worthy of trying. I would get used to the amp in its current form first before making the change, otherwise you may not really notice the effect.
Also I notice in your picture you are running two cabs with your V. Running two 8 ohm loads on the 4 ohm taps also reduces the treble issue to some extent. What I found using CH2 in crunch mode with the gain dimed, mids set to 2pm, bass at 10am and treble at noon or lower I was getting an awesome heavy metal tone that I could not duplicate with any other amp including the Roadster. That alone was one of the main reasons I did not get around to selling the amp since I would definitely miss that for sure.
