Hi Ants, I would say that the Flatcap is worth keeping, but at 20 years old it will certainly benefit from a recap/service if this has never been done. The same might apply to your power amp.
I’m a huge fan of the NAC152XS. I have my Flatcap XS on the preamp’s Upgrade 1. I agree with ChrisSu that a service of both your FC and NAP150x should be worthwhile.
Powering the NAC152XS from your NAP150x (ie. without the FC) may give you an idea of the FC’s condition.
Good to read that you have the units on three shelves.
I’m enjoying them so far, I have upgraded from an old Yamaha av receiver. Before that was using an old dpa integrated amp (which to be fair was pretty nice).
I’ve been able to get the NAC152XS using the AV unity gain with the receiver and have been very impressed with that compared with the receiver only.
I think my plan will be to get a 5 pin link plug so I can remove the FC, get that serviced and then maybe look at getting the NAP serviced too a bit later.