Hi
I recently had the opportunity to listen to Lake People products at an audio fair, at the Meze booth (so, not with a LCD5, but with my test tracks via Tidal). In particular, the Niimbus US5 (with a Gold Note DAC) and the DHA 380. I found them to be excellent machines both in terms of sound and construction, the former technically superior (and given the price...) and neutral and linear, if I must say similar to the Benchmark. The latter had a more "easy" sound, a touch more body and warmth, softer in definition, perhaps limited by the internal DAC. If the 550 is somewhere in between, it could be a great solution.
1) Yes, I find that the LCD5 is truly excellent in rendering "environmental spaces", the ambience with its reflections. Honestly, with equipment at its level, I think its soundstage is also adequate in terms of size
2) It's very simple in reality, by enlarging the space painted by the soundstage, it also enlarges the perceived dimension of the sound source. In practice the Luxman magnifies the three spatial dimensions but the directionality and layering is also vaguer and less point-like. I'm obviously exaggerating to be clear. It depends on the musical genre, but overall the great sonic openness of the Luxman is remarkable. Yes, it's big. Big and heavy, you can consider it a 45x45 cm square, if you include the knobs... absolutely not desktop-like.
3) There is a major conceptual error about HPA4, namely its supposed aseptical nature. It's actually the perfect wire-with-gain amplifier. If the source and recordings are poor... well the result is crap
If they are excellent, the result is remarkable. For example, the latest Peter Gabriel via Auralic Vega G1 is a mind blowing listen.
Now, obviously it is full of poor recordings, this is why I myself have a two amplifier solution, one of which is more "permissive" and friendly like the Luxman, which even with non-optimal incoming signals, puts its own personality into it... a "sound doctor", let's say
So the Benchmark can be fabulous or boring depending on the circumstances (I would never couple it with Hifiman anyway, notoriously hot in the treble range...), its volume control with 256 steps / 4 channels for hps and speakers is crazy tho, perfect even for IEM.
I don't know if Topping amps are boring even with quality sources and recordings, and are bottlenecks themselves: there must be a reason new ones come out every month.