Telos. No the ModWright does not at all lack bass. It's a great preamp that is not at all thin, overly tubey or a wimp.
I spoke to Pass when I was looking for a new preamp and they strongly cautioned me using a singled ended preamp on the X250; I know on my former preamp it was too noisy. The X250 has 30dB of gain so it's a little noisy to begin with. They .5 and .8 series have had their gain dropped probably for that reason. They mentioned to me they ran it that high for home theater users to begin with.
I never had the XP10 at home but came close to trying it. Read a lot of reviews from actually users using it, also X250 and ModWright users and came across several here using the X250 with MW LS36.5 so I went that route and since my former preamp was a ModWright I felt safe and it is a great combination.
It is a fine preamp but like any tube pre, tubes make a difference and I am running none of the stock tubes which seriously made a great difference for the better.
Whatever you try or end up with I would stick with a balanced unit.