Seems to me the velocities were 'supposed' to be closer to 1,600fps with the 650gr. 'top-end' load.

You may have to try Swiss 1 1/2F powder - which will get the speeds up a bit.
GOEX Old Enysford might be another choice for testing.

In that large case, I'd be tempted to try the 3031 or 4895 for the duplex, in 15gr. to 20gr. weight - it certainly isn't as volatile as the faster powders and may be of more help in this instance. I would also load as much BP as I could over the smokeless 'kicker' to create a compressed load with the bullet or top wad- say 1/10" compression. You don't want to compress the powder enough to 'meal' it.