I don't agree that you won't hear a difference between the 30 and the 5. The 5 goes a bit deeper, is more resonant with male voices and has a larger presence in the middle. I used a CSi3 for years. It was a good speaker, always did the job. Then I replaced it with a CS400i, an older, larger center that was a closer match with SDAs. It was apparent immediately that the 400i could keep up better within the huge front soundstage. So, you'll notice, yes. Will it blow you away, probably not. It will be an improvement.
In regards to amps/power. Yesterday I picked up some LS90s, a tall line array from the mid 90s. I auditioned them at the sellers home on an AVR. Very nice balanced sound, but very little bottom and no dynamic weight even when cranked. If I didn't know anything about these speakers I would have passed, but I had bought their little brother the LS50 for my son a few years ago, heard them on my rig and knew what the LS90s were capable of. So I brought them home. I power my fronts with a Carver TFM45. It's a 2 channel amp that's capable of 375watts @ 8 ohms and a bunch more @ 4 ohms. I'll never need to use that much. In fact, when testing, 150 watts takes my 1Cs 3dbs above reference level. The rest of the power available is dynamic heardroom. I'm digressing...
Anyway, brought the 90s home and hooked them to my rig. At moderate volume, I now had weight and impact to dynamic peaks, bass became very tight and punchy. In short, I heard the speaker at it's best. LS90s are an easier load than your 12s. I'm sure you'd hear an improvement even at moderate levels when or if you do the pre/pro -amp thing.
