[shovah]

sdr dosnt stack, i dont think anything does. to get a stronger blood mage use your pretender or use boosters (in my test game with DF marignon i had a blood7 fire 4 goetic master).feel free to empower and level or 2 when possible and get your blood economy running fast (standard should be 3 diabolists with SDR's in a 5000+ population province with 0 taxes, try to repeat this set-up 3-4 times.
make soul contracts with you stronger mage(s) when possible as the 1 devil per turn is alot better when you have 5 of them (all traveling with your army to make up losses). i personally dont like harlequins unless you need to mass fodder quickly, 4 devils will probably be better in alot of cases due to higher stats and heat radiation.
casting the looming hell with alot of slaves gives you a nasty little global that can be very annoying to say the least and a very nasty tactic ive discovered is 10 (or more if you want) assasins with lifelong protections and possibly amulets of luck and robe of shadows if the enemy has lots of crossbows or something. while 1 lifelong protection is weak in battle, 10 of them produces a huge number of imps EVERY TURN, with each of them having 2 attacks per turn they are very good at engulfing your foe in a tide of red midgits. the assasins add mobility with their stealth and utility with assasinations but they are by no means neccisary, its perfectly possible to do this with any commander (the bonus of this way being that you have arrow catchers)