That is an easy one for me. I have been blabbing on A’gon about which component I think is the real giant killer. It is the LA4 without a doubt. I say this because I think the LA4 can be used in all scenarios I have personally encountered. I do not mess around with tube amps so I have no clue about compatibility there.
If your preamp has some noise, and all the preamps I listed above do have some noise. My personal feeling is that the LA4 would be my preference in all of those systems. The unit does not look that nice so I can understand people not being interested in it. However, if you are looking for total silence from a preamp then this is the unit to try out.
I guess the question to ask yourself is, what do you want your preamp to do? As I said earlier, what I want from a preamp is a great volume control (how is it al low volume), multiple inputs for my 4 sources, and silence in the circuitry to not alter the sound in audible way. That is the HPA4/LA4.
The HPA4 also made me want to spend close to $3K more to buy an incredible headphone (Meze Empy), which I I love. I used to hate headphones and only bought the HPA4 instead of the LA4 because of reviews stating it was one of the best headphone amps in the world (for $500 more than the LA4). So I am listening to more music today at all hours of the day with the HPA4. That is a good thing.
The AHB2 is good or great depending on what you want from your amp. I can see 50% here liking it and 50% not. However, my perception of what a preamp should do makes me think most people would like it. Maybe my perceptions of a preamps role are not shared by others, I don’t know. I live in an audiophile desert, except for A’gon.
I have some new speakers now (Thiel CS3.7) and I am looking to get a second amp in the future with a warmer sound signature (with a caveat). The Luxman m900u (which I love) or the CODA #8 or #16 which I need to demo. The AHB2 is very good with these 2 Ohm speakers but it may not be great. The AHB2 was great with easier to drive speakers (KEF LS50) and I cannot use the AHB2 in mono for these new speakers based what I hear is feedback from Benchmark themselves.
Before I get a second amp (the caveat), I am working with a sound engineer to apply Digital Room Correction to my room (for DAC streaming only not analog sources). Going to be starting that project today with the microphone I just got. It will be done using Convolution file(s) running on my ROON Server.