Ah, got'cha.
As you know, the 434 is limited on power. If you wanted to rock your block you'd need a very efficient speaker. Klipshorns would do well. Of course a pair of them would be as big as a new MINI and cost as much as a used one.
It's an annoying bit of physics that smaller speakers tend to be less efficient then larger ones. But since you won't need to crank them up to eleven you should be able to pair your 15wpc something smaller.
Ideally, I'd shoot for something with sensitivity of 90dB (1W,1m) or greater but smaller speakers that sensitive aren't too common.
IIRC the Minimus 7 is around 85dB, not great, but pretty good and more than adequate for moderate listening levels.