Unless your planning to do separates, that is a preamplifier and amplifier what you may be looking for is an integrated amp. That is a receiver minus the radio.
Ratings of watts of a speaker are more a guide as to how much to limit yourself to. Will playing a 40 watt amplier loud equal a 120 watt played lower? Maybe. How loud do you listen to music? Are we talking rock concert levels or maybe chamber music levels? More power is more power but is it needed is something you have to ask. Just how loud do you want that crescendo to be? How dynamic do you want things. Maybe the best bet would be to see what a 40 watt amp sounded like in your system and go from there. A good audio store might let you have one on a demo to find out.
Ratings of watts of a speaker are more a guide as to how much to limit yourself to. Will playing a 40 watt amplier loud equal a 120 watt played lower? Maybe. How loud do you listen to music? Are we talking rock concert levels or maybe chamber music levels? More power is more power but is it needed is something you have to ask. Just how loud do you want that crescendo to be? How dynamic do you want things. Maybe the best bet would be to see what a 40 watt amp sounded like in your system and go from there. A good audio store might let you have one on a demo to find out.