First, your english reads fine to me.

Second, moving from the amps built into that Yamaha receive to a separate power amp (and I say this thinking of any of the three amps on your list, or even the little 7075) would be a nice upgrade.

Since your speakers are DIY, I'm not terribly sure what the overall efficiency of them might be. The RX-V1400 RDS is rated at 7x110W, although I don't know how that rating was achieved (may have been with just one channel driven). My suspicion is that a 7125 would be a nice "bang for the buck" upgrade. I went through a similar upgrade process a number of years ago, from Yamaha receiver to Outlaw receiver to Outlaw receiver plus separate amp, and each transition was a rewarding upgrade for me.

If you were to add a power amp, would you be planning to retain the Yamaha as a volume control or would you be using the HTPC's sound card software as a volume control? I'd prefer a component like the receiver for that role, but don't know enough about HTPC's to know what the practicality might be for going straight from PC to power amp.