Hi Bob: 

Do you use an auto tuner with the als-1300? 
Using the 590's tuner, isn't that just tuning the input of the amp? 
That's how I always thought it was, turn off the rigs tuner and use a tuner on the other end to tune the antenna to run an amp through. 
Considering the ALS-1300 doesn't have a built in tuner, just foldback protection, I think your antenna would have to be pretty much cut to resonance or at least down to a 3/1 SWR on all bands. 
Do you see any gain on 6M this way, how much power out do you get?

73: 

Michael De VO1RYN