I apologize to the group for going overboard on stuff I should never have talked about. I need to meet with my accountant about the joys of 50 states of tax nexus and I am trying to avoid anything that bores me.
I will answer this question only from my perspective and PrimaLuna. If you have a budget of $20k for amp and speakers, that's $5595 for an EVO 400 Integrated and $14,405 for speakers. In some cases an EVO300 is just as good, so if you got that one add $900 to your speaker budget.
If you bought an EVO400 preamp ($5295) and power amp ($5295) that would leave you $9410 for speakers.
Assuming that's your budget I might suggest integrated and better speakers. That extra $5k can take you a long way, and speakers are hugely important.
If you are BUILDING, then it may different: Yes, having a preamp is better, and our amps are not the exact same as the integrated amp because they have a different design and different transformers. The integrated amps have a 4ohm tap and an 8ohm tap.
The power amps have a 4, 8, and 16ohm tap to more closely match a wider array of speakers, and that's important.
The cool thing is if you add another power amp something happens: You have dual-ouput transformers, one for positive and one for negative side of the signal. This is not common because it can cause problems. Our tube engineer Marcel Croese created countermeasures that allows it and increases sound quality.
It's incredible when you monoblock any PrimaLuna. It's quieter. Output impedance drops, and that means better bass. Instead of the speaker taps being 4, 8, and 16ohm they are 2, 4, and 8ohms. Magnepan and Martinlogan owners love it. It's the reason that Martinlogan has used PrimaLuna at shows in the past.
But in my opinion sometimes people are over-amped and under-speakered.