I'm going to throw a monkey into the scientific wrench this morning. After last night, I've got 4 sets of singles on Wilson Gut 16 mains with Ytex Pro Tour 16L poly crosses at 50/45 in the Prince Exo Tour 16 x 18. I don't see any extra spin with this setup. The strings don't move a whole lot which is nice. I see the videos of the science of snapback and it makes sense but I just don't understand why I'm not seeing it on my shots.
It's really hard to say. But, starting with the knowns, let's see if we can't concoct some hypotheses:
Knowns:
1. Mikeler likes multis
2. Gut/Copoly is generally considered close to as good as full copoly for spin generation. Some people think it's better.
3. Mikeler says gut/YTex didn't give him any more spin, but didn't tell us what he's comparing it to.
4. YTex Super Tour is a copoly. RSI playtesters gave it a 3.7 for string movement, suggesting that it's pretty slippery.
5. TWU friction testing shows that gut mains with pretty much any copoly cross is more slippery than almost any copoly in a full bed. So we can assume that Mikeler's gut/YTex setup was slippery enough to slide and snapback and thereby generate extra spin.
6. The strings "didn't move", suggesting that the gut mains were indeed sliding and snapping back into place.
Hypotheses:
1. Mikeler is wrong. He got extra spin but, because he does not have a tracking computer, radar gun and tennis physics computer as onboard equipment, was not able to correctly apprehend this fact. The only way to falsify this hypothesis would be to pack you off to the Wilson Spin Lab.
2. The main strings were sliding and snapping back, but that snap back was not generating extra spin because the tension was too low. In such an open pattern, the loose natural gut was able to slide and stretch very far out of line, too far. So even though the string snapped back into line, that snapback did not occur soon enough or fast enough to put extra rotations on the ball.
3. The main strings were sliding and snapping back, but that snap back was not generating extra spin because the tension was too low. The effective length of the crosses in a Prince Port racquet is longer than it would appear. Prince racquets already have long crosses because they are so wide at 3&9, and the fact that the string effectively "starts" on the outside of the frame rather than at the grommet opening, makes them even longer.
TWU experiments on open patters showed that the stiffness of the cross strings is critical to get the type of snapback that creates spin and doesn't just result in a high rebound angle. Longer strings tensioned to the same tension as shorter strings are less stiff. So your cross strings were not stiff enough. This can cause the same problem described in hypothesis #2 - mains move too much. Too much main string movement can create a torque called a normal force offset, which is an anti-string torque, that cancelled out any additional spin created by the snapping main strings.
4. You strung too low for such an open pattern, creating an average stringbed stiffness low enough to allow the ball to dent the stringbed so much that a normal force offset large enough to cancel out any snapback-generated spin. Even with strings that don't slide and snapback, but are flexible enough to deform and "pocket" the ball a great deal, this can happen.
5. Some combination of Hypotheses 2,3,4 & 5
To test these hypotheses we need only restring Mikeler's 16x18...
...with a stiffer copoly cross string at the same tension
...with the same strings but up the tensions to 60/55 or 55/55 or 56/54
I would be more confident in these recommendations if you were using a cross string that is in TW's database, as stiffness is a critical factor in the last four hypotheses. Instead, you're using some weird brand about which we have no objective information.