I recently started diving doubles earlier this year and only have about 25 dives in them, so take the below info for whatever it's worth.
A huge part of doubles diving for me, so far, has been figuring the setup out - I feel like I'm mostly sorted out but still learning. I have Worthington X7100s which are about the same height as your tanks. However, the Fabers get a couple of pounds lighter as they empty and seem to be more butt light as well. I'm 5'11" and multiple people told me that I would have trouble trimming out in these tanks as they're "too short".
- As suggested above, use a tail weight on the lower bolt instead of ankle weights. You can also try the DSS extra long tail weight pouch instead of a V weight to get the weight even lower.
- You could move your bands up a little bit. The spacing obviously remains the same - 11" is standard.
- I use a DSS Kydex plate which has the same buoyancy as an AL plate. This takes 5lb off your upper back and allows you to move it lower. I don't think I would have been able to trim my doubles correctly with a steel plate. Honestly, I think this is the main reason for your head down trim.
- Loosen your shoulder straps and tighten your crotch strap. I try to keep the tanks as low as possible while still being able to reach my valves and perform a valve drill.
- Ideally, you're not adding extra weight to trim correctly. With doubles, that may cause you to be dangerously overweighted.
Make small changes and only one at a time. You'll be surprised at how much difference they make individually. For reference, my current setup is 2x Worthington X7100, Kydex backplate, 5lb tailweight, 2.5lb long bullet weight on my belt.
Hope this helps.
