Are you not using a subwoofer? If not, I think I would first invest in a decent subwoofer, which will make a bigger impact than swapping the Monitor 30's for the 40's. I used to own the Monitor 40 SII speakers and they will give you a small bump in bass output over the Monitor 30 SII. Might be a better idea to get a more substantial upgrade when and if budget allows. The Polk Signature line would be a substantial move up in overall sound quality.
Subs>RBH I-12e. HSU VTF-15H mk2. Monolith THX 12(x2), THX 10(x2). XTZ 1X12. SVS PB-1000(x2). Speakers>JBL Studio 270/235c/230/225c. Sony CS3/CS8/CS5. QA 3020i/3090Ci, 2020i/2000c. Monolith Cinema 5bs. Polk RTiA1/CSiA4. Other Audio>Sony MDR-Z7m2,V6, XB900n,700,XB72,XB32. Sennheiser HD58X,HD4.50,PC37X. HiFiMAN HE4XX. Philips SHP9500. JBL PartyBox100(x2), Xtreme2,Charge4,Link300. Sivga SV004. LG FH6,RK8,RK7,PK7,PK5. HK Onyx Studio4(x2)