Hey Reza...
Funny you brought this up - I did the same thing, wondered if my Ch.1 was always a little "gritty" or if this was something developed over time and use. I came to the realization about 2 weeks or so ago, that I couldn't "get" a clean from my Ch.1 [like I think I was at least able to back when I first got the amp]. I used a variety of guitars, variety of pickups. Gain at 8 - 9 o'clock, all settings at noon, guitar on 10/10, MV at 10 - 11 o'clock, channel volume at, say, 10 o'clock, hit a chord hard, it still distorts - gets gritty.
ANYWAY, Poppa recommended considering a preamp change out. I haven't done it yet. But I know when I first got this amp, it was piano-percussive-punchy cleans, no grit. So I'm there with ya Reza, something's shifted.
Peace,
V.