In very general terms & IMHO, from my 20+ yrs as a tube-a-holic, what you MAY hear in terms of sonic differences (all system & tube dependent, of course) when you lower the manufacturer recommended bias from any tube amp is :
- lessening or tightening the impact of the mid-bass
- slight decrease in deep bass.
- lessening of mid-range (vocals) fowardness
- focus and imaging may improve, but at the expense of a hardening in the mid-range as you go lower in the bias.
- soundstage may deepen as bias lowers.
In some systems, lowering the bias may improve the overall sound. Again, your "mileage may vary" and is contingent on your system match-up and tube types/brands.
Happy Listenin'