Why do you need so much power?

It depends on the speakers, how loud you want to go, how large the room is and how far back you sit.
Also the recording.

It’s like driving a car. more horsepower makes for a faster and more responsive ride on any road.

Clipping is hifi public enemy #1. Avoid at all costs. Extra power rather than not enough is your insurance policy against clipping at a minimum.

Thanks everyone. Since my room is of moderate size and I listen maybe 2m away, the 25w J2 is more than enough. To me, clarity, transparency and imaging is more important than volume. 

Volume and those things are not mutually exclusive but more power generally means more cost so need to know where to draw the line.  

No, for modest listening rooms with average efficiency speakers (90 dB or more) even 100 Watts seems overkill.

What will cause you a problem is speakers that dip to ~ 3 Ohms or lower, especially in the midbass, but sometimes in the treble (ESLs). 

It is very hard to find an amp ~ 100 W with great current drive.

I’ve hit clipping on 93 dB/Watt Tannoys (Kensington SE) with 25 tube Watts / ch, though only on certain material at higher volumes. And that’s hard clipping; soft clipping sets in well before that, so more overhead than you think you need is often a good thing for maximum clarity and dynamics. I heard that difference as I moved up in power amps.

I’ve even hit hard clipping once on those same Tannoys with a more powerful 65 - 70 Watts/ch tube amp, though that was a bit by accident - an extremely high dynamic range Sheffield Labs (direct to disc) record of Firebird Suite caught me well off guard! I ended up with 250 Watts/ch Rogue Apollo tube monoblocks and it was truly wonderful, and never ever clipped, even in "whoops" moments.

Now I run 96dB Tannoys (Canterbury GR) with 200 Watts/ch tube monos (VAC 200iQ) and there is really no possible way I will ever clip those lol. It sounds absolutely wonderful.

Of course, if you’re listening to rock/pop with almost no dynamic range (nothing against it, I enjoy a lot of it!) then yeah that 93 dB and 25 tube Watts will run you out of the room and you will never even reach clipping before your ears melt off.