Many on this forum will disagree, but if you are going to use a 20-35 watt amp, I would recommend that your speakers have a sensitivity rating of at least 95 dB. I you go lower you will lose the pleasure of dynamic range. (The range of sound intensity a system can reproduce without compressing or distorting the signal.) With speakers in the 90/dB range your amp will clip on the loud passages and your recordings will sound flat.
HG
HG