Add external amp to integrated pre outs or not?

I have a Simaudio 600i integrated amp. I really like it and want to keep it. Although, thinking about purchasing external amp to run my power hungry Ketemas. Yay or Nay? Advantages/disadvantages?
Not very familiar with your gear specifically, but in general, based on the amp and speaker specs I found,I would say: maybe.

Frequency Response: 26Hz - 33Khz, in room
Nominal Impedance: 4 ohm (minimum 3, 2 ohm at 70 Hz)
Sensitivity: 90 dB/W/m
Minimum power amplifier: 20W per channel

Your amp is no slouch at 125 w/ch and 30 amp peak current.

Unless you feel the amp is challenged to go as loud and clear as desired, there may not be much advantage with any but the biggest and best (and often also most expensive) amps out there.

But if it does, then a separate power amp with -2-4 times the power and similar or better current delivery capabilities would likely make a clearly noticeable difference.

There are some very nice sounding high efficiency Class D amplifiers out there these days that might meet those specs for modest cost worth considering.

Otherwise, you are probably looking at fairly big, heavy power hungry and expensive Class A/B amps to do better.

Off the cuff, those speakers do not appear particularly tube amp friendly, so the big boy tube amp that might be needed to clearly do better would also likely prove to be quite large, power hungry and expensive.

The other factor that could add benefit is that not using the integrateds power section may yield some improvement due to lower noise and/or distortion levels when used more modestly as pre-amp only, but that alone may be hard to bank on clearly.
Mapman, thank you for taking the time to analyze my system. I really need a honest analysis. I am considering staying with Simaudio and upgrading to the W8 by Simaudio (2x the power). I have listened to the Diavalet Class D, but not on my system. The sound was very crisp and clear, but a little to digital sounding. Although, I listened to a CD and not analog side. This is the battle I have had. Will I get a 10,000 difference by upgrading. Difficult to find a dealer that will loan me the amp in order to listen to it. I am not about to drag the Ktemas in for a listen. Again, thank you for your feedback.