Warning: I'm an engineer and I'm over-thinking things a bit here, but I'd like some advice in full recognition that free advice is worth what you pay for it

Background: Wife and I have 10-12K miles on the tandem. We've been riding for 4-5 years. Wife only rides the tandem, while I've been putting in a good deal of miles on my single; have gotten a good deal stronger over the last 2-3 years. We do a bit of riding during the week, a longer (30-50 mile) ride on the weekends and 100K organized rides whenever we can. We both enjoy riding the tandem, but I'm finding it increasingly difficult to find a cadence on the tandem where I can "relax". Of course climbing is much harder on the tandem, but that's not really the issue.

Single bike analogy: On the flat, no wind, working hard you can find a gear/cadence that feels comfortable, sort of a cyclic load on your leg muscles to which you are best adapted. Introduce a headwind - you decrease speed, but it becomes very difficult to find that sweet spot. The tandem feels like this unless we are a) headed downhill or b) have a tail wind. I'll shift down on the tandem, but continue to find the effort much more fatiguing than on a single.

Theory #1: The headwind creates a condition where you can't match the conditions to which you're best adapted; same thing seems to happen on the tandem albeit for a much greater percentage of the time.

Theory #2: I'm a grumpy old guy who should HTFU.

Is this in my imagination? Anyone else experience this?

Next Steps: I changed from 175mm to 180mm cranks on the tandem to lower my cadence. This worked, reducing my cadence 2-3 rpm, but I'm thinking of going back to 175 to match my single. I didn't really notice the change, but the additional 10mm of diameter could make enough of a difference in range of motion to be a contributor.