Minimum Power Needed to Drive Klipsch Cornwall IVs? - Considering Erhard_Audio Ray

Way more than enough.

SET is the way to go.

I have the Ray in a secondary system driving Omega monitors in a larger room and it has plenty of punch. I never felt it lacking in wattage or limited in that regard.  

I use the built in passive pre and have done some upgrades with Miflex coupling caps and upgraded binding posts/RCA's.

This is a beautiful little chunk of an amp.  Louis at Omega finished my monitors beautifully in Padauck to perfectly match the cabinet of the Ray.  As you know the Ray uses very high quality Lindahl transformers.  It's sound is very neutral.  It is not slow, does dynamics well, images beautifully and has good "color saturation" if you follow my analogy.  Many affordable tube choices.  Highly recommended.

For 102 dB sensistive speakers, one watt will give you 102 dB of volume at one meter in front of the speaker. The volume at your listening position further back is a bit trickier since that depends on the size of the room and how it is furnished. Using two speakers (stereo) to the equation will add a couple more dB to what you hear.

Question -- have you bothered to buy a sound level meter or add a sound level app to your phone?  One's person's "loud" is another's "medium" volume, so its a lot better to get a meter and find out for yourself just how loud you like to listen.  These days I find 85 dB in my room about as loud as I want to listen so I'd find 4 watts with a 102 dB sensitive speaker way more than enough. Still plenty of power left for peaks.

@katokaelin 4 Watts won't be enough to really fill that space.

Here's another problem: if you want to really hear what an SET does, you really don't want to drive it past 20-25% of full power, else distortion causes the amplifier to sound 'dynamic' due to how our ears interpret that distortion. That distortion (higher ordered harmonics) also causes the SET to sound louder than it really is- the mark of a really good system is that it won't sound loud when it really is.

So that means with 4 Watts you really only have 1 good one. For this reason and the size of your space, I'd consider a lower powered PP amp since 15-20 Watts probably will be enough, and most PP tube amps have at least 90% usable power rather than only 20-25%.

Going with a higher powered SET really isn't practical. Above about 7 Watts total output, bandwidth really starts to be a problem because that's the Achilles heel of output transformers for SETs, plus the price goes up exponentially and we're talking about a 45 Watt SET to keep up with a 15 Watt PP amp (again, if you really want to hear the best of the SET).

You might not be able to play your system at very high volume levels, but, for the VAST majority of the time, 4 watts will be plenty.  Most of the time you will not be playing above a quarter to half a watt, and even if you occasionally push toward the 4 watt level, this would probably be only on peaks, and if there is some distortion or compression, that is just one of the compromises one makes with ANY component choice; none are absolutely the best under all circumstances and conditions. 

I personally like the sound of 6L6 tubes, although I have never heard them used as a single ended triode tubes.  Given the low price of the amp, it would not be easy to find other higher power alternatives to recommend.  I have not heard the amp you are considering so I don't know how it would compare with your Cronus amp.

Even if you have little skills at building gear, you could probably build an Elekit 300B amp; the kit comes with very good instructions and you can learn basic assembly skills on Youtube.  The amp would cost a bit more than $3,000 once you add in the price of the 300B tubes (the kit comes without tubes), but, I've heard that amp and it is pretty good, and puts out a little bit more power than the amp you are considering.