I don't think double the power output at 4Ω over 8 is much of an indicator of how good an amplifier is... sure it's easier to make one that doesn't double but there's no reason why one that does would sound better.
My advice would be not to go for an amplifier that's more powerful than you actually need. Most if not all amplifiers sound better running at the top half of their rated output so if you go for something four times as powerful as you need then the cost of that unused power is a reduction in sound quality. The other thing is that most volume controls sound worse the more you turn them down as well, with the exception of decent resistor based attenuators. I did a short blog post on amplifier power vs loudness that might be interesting.
My advice would be not to go for an amplifier that's more powerful than you actually need. Most if not all amplifiers sound better running at the top half of their rated output so if you go for something four times as powerful as you need then the cost of that unused power is a reduction in sound quality. The other thing is that most volume controls sound worse the more you turn them down as well, with the exception of decent resistor based attenuators. I did a short blog post on amplifier power vs loudness that might be interesting.