The Hegel 390 (not 360) would be a top choice to keep things simple. I have heard the Hegel 360 with KEF Blades (similar to the Reference line) and it was surprisingly good given that the Blade is a fairly big speaker. The Hegel 390, which I have not heard, is supposed to have a better DAC and other improvements on the 360. However, the 360 can be bought used for your $2.5k price.
The Mark Levinson 585 is amazing. It costs $12K but can be found used for $6K (even dealer demos). Best integrated I have heard, even preferred it to the Luxman 509x integrated. It would sound great with the KEF Reference 1. I heard that integrated with the Magico A3. The new ML 5000 series is supposedly a little lower in sound quality to the 585 (dealer feedback from dealer who does not sell ML).
I am going to buy some new gear for my KEF LS50’s. I should say old gear because I at one time had Benchmark AHB2 + DAC3 HGC on the KEF LS50’s but sold them. I want to buy the AHB2 and the new LA4 preamp again because I enjoyed that clear, silent background from the Benchmarks. Not sure if you would like that but if you did go that route I would get 2 AHB2 and run them as monoblocks. They do have a 30 day home trial. https://www.stereophile.com/content/recommended-components-2020-edition-power-amps https://benchmarkmedia.com/products/benchmark-ahb2-power-amplifier
I have also considered going out and listening to the following to pair with the KEF LS50’s.
- Bryston 4B3
- CODA #8
- Hegel 390
However, the AHB2 is the bird in the hand so to speak and I likely won’t bother getting my butt out the door to go demo them.
BTW - I am currently using a Peachtree Nova 150 with my KEF LS50’s. It is rather good, not the best you can get but if saving $$$ is the #1 factor, a very good choice. For the KEF Ref 1 I would consider the NOVA 300 or the new NOVA 500 with the improved DAC.