Fairly sure the FatMat is butyl based and not asphalt based... but I could be wrong.
There is nothing wrong with Car Builders sound deadener at all, I was more leaning towards this myself at one stage, considering it is a true butyl compound. By the looks of it (from the quick research I just did) FatMat comes in asphalt based and also butyl based. It's not very clear in the descriptions anywhere.
You opted for Car Builders butyl based sound deadener, which was a great choice.
2 channel 50W RMS amp? hmm, well a 100W RMS speaker pulling from a 50W RMS... rather underpowered. IMO you are better off purchasing a matching (or higher) wattage amp for your speakers, as you can then turn down the power on the amp itself to match/balance your speakers so you don't risk popping or melting them, and you don't risk burning out your amplifier.
100W RMS per speaker, 200W RMS, plus your 60W RMS per speaker, 120W RMS... so a 400W RMS amp would suffice. Something like this would be perfect to run all 4 speakers. 4 ohm and 2 ohm stable at the same wattage.
I'm not sure on what $$$ you are intending on spending... but a less wattage amp would do... but it wont be able to power all 4 speakers to their full potential. You could probably get away with running cheaper amplifiers also... like one of these or two of these. In saying that, your best bet is to match an amplifier to your RMS wattage of your speakers, no disappointments then.
*EDIT, even something like this would suffice.