Tom,
I'm using a diffusion head. I'm very confident in my metering. I couldn't imagine the presoak being responsible for such dense highlights. My agitation is normal to what I use with other combos. I've never heard about slow agitation with rodinal.
I've found out that proper agitation is a important factor for success with Rodinal, with my best results coming from gentle inversions and longer intervals between those inversions. I haven't nailed down Delta 100 yet, I don't shoot it that often, but Acros souped in Rodinal is AWESOME

I dilute Rodinal 1+100 and devlop it for 18 minutes at 70° F. I start out with thirty seconds of gentle inversions , 2 seconds for each inversion, then 2 inversions every 3 minutes for theremaining time, followed by a 2 minute running water rinse and regular fixing, hypo clearing and washing proceedures. When I get everything right, eposure, focus and printing, the results can be stuning., even with 35mm and a condenser head.
Mike Sullivan