Hate to say it, because I own some of their IR's, but they suck.cab IRs from 3 Sigma Audio
I thought Positive Grids BIAS was really good. My best real/sim combo is a 6506MH's line out into the interface and using NadIR with various combos of mesa/orange IRs.
Not my intention to divert this post too much, but I thought the line out from the 6505MH had already IRs loaded into it, that you kinda "had" to use?
I just bought one and haven't had the time to read the manual yet.