I had the Polk S30 paired with the large S20 bookshelf speakers and never had any issues. I think it would be obvious since you have timbre match issues using the Arena 125c that you first try the S30 center, which is the one dedicated center channel in the Polk Signature line.
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 Studio 530/520c, 270/235c/230/225c, Arena 130/125c. Jamo S807/S803. Pioneer FS52/BS22A/C22. Sony CS3/CS8/CS5. QA 3020i/3090Ci, 2020i/2000c. Monolith AM Cinema 5 bs. Polk RTiA1/CsiA4. Infinity R152. Other Audio>Sony MDR-Z7m2, WH-XB700, XB32, XB31.RBH EP1. Logitech GPro HS. Plantronics RIG 500 Pro HS. LG FH6,RK8,PK5,PH4. UE Boom3.