I had 4 K Blade 98 last year, and I know some pro players use a poly in the mains (often some kind of Luxilon string) and then natural gut in the crosses - but in my honest opinion, natural gut in the crosses is a TOTAL UTTER WASTE.
What you look for in a hybrid with a poly in the mains and a softer string in the crosses is for the soft cross string to loosen up the stringbed slightly, but you never really get to feel much from the cross string.
90% of all the feel, power, spin and a good deal of the control comes from the main strings, so I would look for a Prime Multi as your go to cross string in a hybrid with a poly in the mains.....
I ABSOLUTELY LOVE natural gut - and I use it in 4 of my racquets, but ONLY as s main string in hybrids with a poly in the crosses or during the 3 coldest winter months here, in full Natty gut job.
I did however try Natty gut in the crosses and Luxilon alu BB power in the mains last year, both in my K Blade 98 and in my 4 Babolat Pure Storm Tour GT. While it played nice for 2-3 hours, after that I rapidly started to lose all control and it got worse and worse. So Luxilon Mains and Natty gut crosses is good for 2 hours, then it plays like crap in my personal honest opinion.
Somehow the poly in the mains, when it dies, makes the Natty gut in the crosses overtake all the power, because the Natty gut in the crosses lives on, while the Luxilon in the mains has passed away long time ago.
When you do the opposite, Natty gut mains and poly crosses - the hybrid for some strange reason keeps on playing well even after the poly in the crosses has died, because the Natty gut mains lives on and the dead poly in the crosses just tames the power and livelyness of the Natty gut in the mains,and which makes this hybrid very long lasting unless you break it of course.
I still like to use a good thin spinny poly in the mains, and sometimes I just do a full job of a CoPoly at VERY LOW TENSION, 52 lbs with SIGNUM PRO TORNADO 18 in the mains and SIGNUM PRO HYPERION 17 in the crosses. But I still want to try a softer string in the crosses, and therefore I started to test Prime Multi strings and ended up buying the best Prime Multi Babolat Xcel Power 16, which is in my view the best Prime Multi on the market (better than X-One and NRG2). Xcel Power holds tension well, feels crisp, not too much power even though It's called Xcel POWER and stiffer than a Natty gut string, so in Factor I prefer the feel of Xcel POWER in the crosses with a CoPoly in the mains,and compared to when I used Babolat VS in the crosses.....Believe it or not. This is why I say that Natty gut used in the crosses only is a total waste of gut and money.
However Natty used either as a full job or in the mains is like heaven on earth. NOTHING beats Natty gut here. But Natty gut in the crosses - here I prefer the cheaper Xcel POWER multi actually and honestly.
I just bought 20 sets of Babolat VS TEAM Natty gut, so It's not like I don't have enough Natty gut to experiment with
Luxilon strings are GREAT and used by a lots of pro's, but they die fast and are not mean to be played for more than 3-4 hours, then cut out.
The never CoPolys are better at holding tension and elasticity, so instead of going down the expensive Luxilon route, and only have a good performance string that last you 4 hours, then consider another CoPoly with better tension stability.
Your main poly string should always be in as thin a gauge as possible, because this give you extra control, power, feel and comfort in a poly.
As your main CoPoly string, I would try Signum Pro Tornado 18 gauge, Tecnifibre Black Code 18 gauge, Signum Pro Poly PLASMA 18 gauge or WeissCannon TurboTwist 18L gauge and then skip the Natty gut as your cross string and instead get a cheaper (but still quite expensive) Prime Multi like my favorite Babolat Xcel Power or either of the Tecnifibre Prime Multis - X-One or NRG2.
Since the Prime Multi is for the crosses, and the crosses contributes mainly to comfort and control - go for a thick 16/1.30 gauge here. This stiffens up the stringbed a bit more compared to a thinner gauge multi and keeps the stringbed from playing like a ROCKET LAUNCHER, once the poly in the main strings startside to die.
If you want to try natural gut, which I certainly think you should - and if you're a female player, you get lots of extra nice power, spin, feel and comfort from using natural gut - I would suggest my own currently setup, which I use in 4 of my racquets, a NATTY GUT Babolat VS TEAM 17 @ 62lbs/28kg in the mains and a CoPoly Signum Pro Hyperion 17 @ 58lbs/26kg in the crosses.