Under the same settings, my mark v's clean channel sounds very thin compared to my mark v25. Same cab, guitar. I changed v1 and v3 but same results. Is this by design or is my mark v faulty?
Yes - but the preamp is only half the equation. When you want bigger tone, you generally want bigger headroom, bigger wattage and bigger iron. That's where the power amp part becomes relevant. I can hear a clear difference between the 10W power mode and the 90W power mode on the V. FWIW, I always use the 90W mode on my V because of this; it simply sounds bigger and ballsier than the other modes.
Similarly, if your big V sounds thin compared to your V:25, there's something wrong either with your amp (or its tubes) or with how you set it up.
(Or maybe we are just hearing 'thin' differently.)
I think the main issue is the difference between the tone stack. The treble on the v can't go past 12 uo'clock or it starts to sound thin, while on the 25, it won't sound thin till 3 o'clock. I thought they have the same tone stack