Nope... I meant the VK5i.... NOT 51SE.
the 51SE is going to be very similar to the sound you presently have, albeit a bit richer. A friend of mine has one and he loves it, though he also has a 5i, and they are worlds apart sonically. The 5i having the more classic sound than that of the 51 or 51SE (both of which are preferable to the 50).
I've posted lots of threads on the vk5 i asking about it's sound, tubes for it, etc. check them out. Also there's got to be tons of posts on the 51SE too, as it's a very popular item.... both are strictly balanced topologies, and use xlr connections expressly... although the 51 series does have some RCA on it I believe. I used adapters on mine with fine effect.
Tehn there's that age old question of age... The VK 5i's run stopped back around 2000 or so. So even the last one BAT made is going to be over 6 or 8 years old.... but the build quality on them is superb. BAT says time and again they should last 20 years... and they're all self biasing, and for some, that's a big plus. With your budget, maybe the 51 is a better choice... if you go with BAT. Also their support is excellent, even with preowned gear.
I aimed all my comments on your sonic preffs. A Thor TA 1000 would definitely be a step in the right direction too, or even it's later sibling the MK II, (what I own), but didn't want to mention it as the only support for them since Mr. Marks flew the coop on his following so abruptly, will be via some local or well respected tech.
I mention this just to be quite frank on it all... the build of the Thors is top flight across the board. I've had not one issue with mine whatsoever aside from picking out which tubes to run in it. I've also seen some TA 1000 & MK II, lately for very good prices too. The only remote control feature is volume control however... and the balance control is defeated normally, and was offered as an option, along with HT pass through. 4 inputs. 1 line out. 1 active outs... and all RCA. 4 tubes. 2 12AT's, and 2 12AX's. Self biasing. Outboard power supply... extremely well damped and arguably the most unique physical esthetic of any make audio gear.
The MK II is the quicker of the models. Dynamic, fluid, harmonically natural and full. Great bass. super imaging. Big SS... wide and deep.
The MK IIs serial numbers began at 1000 & up. Previous to that were just the TA 1000. The TA 1000 is what I heard in person and also what made me want to get into an all tube array. It was by far the best sound I'd heard in nearly 30 years and thatÂ’s just my take. It is like a BAT VK5i on steroids. Lots of steroids.
The older CJ's, eg., 10, 9... are reputed to have even a more liquid midrange than the VK5i.