The S35 and S15 will both have a -3dB point around 60Hz, so 80Hz would be perfectly fine for a XO setting. This would be particularly so if you were using the S15 as a front speaker. Setting a XO 100 to 120Hz increases the chance of subwoofer localization, so you usually want to avoid a higher XO unless your speakers simply don't have much lower frequency response.
Subs>RBH I-12e/I-12. HSU VTF-15H mk2. Monolith THX 12(x2)/THX 10(x2). XTZ 1X12. SVS PB-1000(x2). Speakers>JBL 270/235c/230/225c, Arena 130/125c. Jamo S807/S803. Infinity P363/163. Sony CS3/CS8/CS5. QA 3020i/3090Ci, 2020i/2000c. Monolith Cinema 5 bs. Polk RTiA1/CsiA4. Other Audio>Sony MDR-Z7m2, WH-XB700, XB32, XB31. Sennheiser HD58X, HD4.50, PC37X. SIVGA SV004. HyperX Cloud PS4 HS. Plantronics RIG500 Pro HS. LG FH6,RK8,RK7,PK5. UE Boom3.