First of all @bonzo75 made good points. I recommend you take his advise.Hi all ok I have an update here. I have been in touch with Kondo and they have advised the Ongaku was in fact designed as a power amplifier and using a preamplifier is their recommendation so there you go .... The Overture is their true integrated amp.
Kondo Ongaku or Overture has the same route for signal like most tube amplifiers.
Input selector-other switches-volume pod-amplification stages.
So there is no difference in general layout which makes them both integrated or power amp. You decide.
I know Kondo keep saying Ongaku is a power amp but 6 of my friends have Kondo Ongaku and I listened them in various setups hundreds of hours over the years. I heard Kondo Ongaku with or without preamplifiers including all Kondo preamplifiers such as G-70, M7 heritage and G-1000. Yes when a preamp is added to Ongaku it sounds better but it still sounds great without a preamp.
IMHO the answer to your question about buying Ongaku is not related with preamplifier it’s related with speaker. Ongaku is a great amp most probably the best SET amp but only if you can find the proper speaker. Don’t ask me which is the best matching speaker cause I don’t know. Look for something over 94dB with an easy load.
BTW if you’re referring to AN UK Ongaku I heard the top silver model many times with different speakers too. It’s a great amplifier, silky smooth but nothing like Kondo Ongaku.