A lot of people would say for low-bass applications (ie, not subwoofers) not a lot of power is needed and a 140 watt amp would be ok as more power might not be utilised.
I think that's twoddle
My answer would be to go for as much power as you can afford. Speakers are FAR MORE at risk from being underpowered than overpowered!! In fact, as long as the crossovers are set up correctly, huge amounts of over-power will do almost nothing. So just set it up at the volume you want, and enjoy.
I run my speakers off a 600 watt amp - loud enough to hurt, and my subwoofers off a 4000w beast that lurks in the boot. (Unfortunately only 1000w of that is utilised as it is multi-channel and I had no choice, which is why -now that I can afford it- I'm switching to a 2kw monoblock)
That reminds me: for subwoofers, go for Class D monoblock amps. They're pricier, but generate way less heat and are way more efficient, and are usually designed purely for subwoofer applications.