What was the difference between the vanilla A400 and the 'X' anyway ?
not 100% sure, as I have only heard the standard A400 ... but what I have read from many who have compared the two, the standard A400 is by far much better
but as I have said, the standard A400 is very good for the price when partnered correctly ... you will read as many good reviews as bad ones ... I have helped several of my son's college mates set up cheaper systems and many comprised the A400 amp
the best combinations I have heard are:
Pioneer A400 with a Marantz CD63 MKII K1 signature cdp in combination with:
Mission 753 speakers (the earlier ones, not the freedom model) ....
Mission 751 speakers (both the older and the freedom model) ...
B&W DM2 speakers (but have found that the DM2 speakers work better with a stronger amp) ...
the old Quad 11 speakers (but for me, I found bass a little on the weak side)
old IMF ALS40 speakers
I have also heard all the above with a Rotel RCD-965 BX cdp (standard aswell as the limited edition version) and also worked well with the above combinations, aswell as an old Pioneer PD-S703 cdp
for all the above combinations, I found that using decent LC-OFC (copper) speaker cables and a decent copper based interconnect worked well (for my tastes)
bear in mind, that there are most probably better more expensive combinations, but these are the ones I have tried and enjoyed