Do remember that on double wall sockets, the earth usually connects to the L/H side of the socket (where there is provision for such), not the right, therefore for that reason, quite often you'll find that equipment plugged into the L/H side of a wall socket sounds better.
Now, if your mains distribution block is plugged into the L/H side of one of your sockets, then everything connected to the block is taking advantage of that improved connection, which may explain what you're hearing.
It's one of the reasons why I always go for single wall sockets, rather than double, as each will have its own individual earth point. And if you're using a distribution block, you should always plug the most sonically influential kit into the socket nearest the incoming mains (the first one next to the cable powering the block), followed by the next most sonically influential, etc.
The most sonically influential piece of kit, for me, is always your main source (T/T, CDP or whatever), followed by preamp, then power amp. Although, in a vinyl system, a separate phono stage would come before the preamp.
Let me give an example...
Say you've got a CDP/T/T/phonostage/preamp and power amp, and a 6-way distribution block, plugged into the L/H side of a double wall socket, and the T/T is your main source. Then I'd plug the T/T into position 1 (first socket nearest the incoming mains), phonostage into position 2, CDP into position 3, preamp into position 4, and power amp into position 5.
*However* sometimes power amps benefit from being connected to their own dedicated wall socket. Therefore, in your case, with two double sockets, you may benefit from connecting your distribution block to the L/H side of one socket (with your equipment plugged into it as outlined), and isolate the power amp, by plugging it alone into the L/H side of the other double socket.
Anyway, just a few things for you to consider!
Marco.