[Elecraft] Tuning KAT4 into KPA1500?

[eric norris]

My K4 shows about a 1.8 SWR while operating into the KPA1500 on 6m.  My 6m antenna has a flat SWR, 1.09.  But when I activate a K4 TUNE, with the ATU engaged, the KPA1500 stops TX.  There is nothing in the K4 online manual about forcing a tune while the amp is running.
Is there a way to do this, or alter the K4's atu settings manually?  I'd like to keep the K4 as cool and happy as possible.  

73, Eric WD6DBM