Something like Clays if you want clean burn (since it a bulky powder in the first place), or something like bullseye or promo (uses red dot loading data) if you don't mind a dirtier burning load. Hence your looking for a fast burning, bulky powder that will give you a decent case fill.

For Clays, click the below link  and select  44 mag and Clays powders only (leave the center selections alone).
http://www.hodgdonreloading.com/data/pistol

Bullseye,
http://www.alliantpowder.com/reloaders/powderlist.aspx?page=/reloaders/powderlist.aspx&type=1&powderid=1&cartridge=33/

Promo (uses red dot loading data by weight).
http://www.alliantpowder.com/reloaders/powderlist.aspx?type=1&powderid=4&cartridge=33

As for spherical powders, veer way from them for your 44 mag subsonic load, since they are a too dense of a powder (opposite of a bulky powder that take up more room in the case for the same amount of weights) and even with a fast burning power like 231/Hp38, you not going to have enough case fill for times that you may be aiming downward, with all the spherical powder against the bullet/away from the primer flash channel instead.

And trust me, out of a marlin 1894 with a 270gr bullet with about 4gr of Clays, is hilarious at 100 yards, since you can hear the round going down range all the way if you have side berms on the shooting lane.  Hence tried this out for a light load in 44 mag for my 270gr cast bullet, but could not bring myself to keep shooting such a slow bullet taking that long to find the target, and stepped up to universal Clays as my med load isntead.

231/hp38 (same powder) for a 240gr cast bullet, has a start weight of 5.5gr at 800fps at 12K cups (not going to be the cleanest burn due to the lower pressures) and max 11.0gr for 1,334fps @38K cups.

So granted that your safe down to 5.5 grs, but does not have a lot of powder in the case, so downward shots maybe a bit spotty with  off SD's instead.

But to back it up, your looking for a fast burning powder that is going to be near max pressure at the intended speed you want to run.  This will end up with the cleanest burn  from that powder that is possible.

Pretty good burn speed list for powders, so once you figure out the speed you want to run the bullet, easy to dial in a powder that will give you full pressure load for the cleanest burns at the speeds you want to run.

http://www.wwpowder.com/PDF/Burn%20Rates%20-%202015-2016.pdf

Hence the reason for wanting a full pressure load as the speed you want to run.

With a full pressure load, not only does the case seal well off to the chamber walls to prevent soot'd cases, but the cleanest of possible burns as well.

I agree with Derek45, you should visually confirm the powder level in every case you load.