What a small world . . . I too play through a METRO/D115XLT stack, you obviously have EXCELLENT taste in amps!
While I've never owned/played a Series 2, maybe I can offer a general suggestion: I'm going to guess you want a fat sound without bulk and that ALEMBIC definition. From that standpoint I'd run more neck pickup at a somewhat darker setting and add enough back pickup at a reasonably bright sound to round it out. ALEMBIC's traditional layout of the neck pickup at the end of the fingerboard and the bridge pickup at the bridge is a little more extreme than other instruments' placement. The brdige pickup can get real twangy real fast.
I play a long scale SPOILER, updated with twin FATBOY pickups. I have no balancer, just a both pickups on position. As such, I've adjusted the trim pots' mix to be a bit neck-heavy to anchor my sound, but with enough bridge to grab that back pickup definition. I fine tune it by playing closer to the bridge or neck depending what I need at a given spot in a tune.
The other part would be the METRO: I would not add another preamp. David Nordschow has put in tone controls that cover a LOT of ground, to the point the tone can get VERY extreme pretty easily. The Enhance knob doesn't do much for me.
I sometimes will get the basic tone in the SS channel with its 3 knobs, then transpose that layout to the tube side where I can fine tune that handoff with its greater control depth and tube warmth.
IF it were me, leave the METRO in a flat configuration on the tube channel. Get the S2 as close as possible on the bass first. Then start your amp tweaks. Between the endless variations available in S2 electronics, and the huge range of tone available in the METRO, I would NOT expect you to figure this out in one sitting. I would not be surprised if it took weeks to find your sweet spot, as this is WAY more involved than slapping a Jazz into an SVT.
Of course the payoff is a tone like no other, impossible to achieve with anything else.
'A little thin in spots' is usually cured by some push in the 100-300hz range, or more neck and less bridge pickup.
People are often a bit surprised that an ALEMBIC doesn't sound as 'bassy' as a lot of other instruments. I liken this to the first time most people hear an 'Absolute Sound' level, high-end audiophile stereo set-up: It sounds so clean because 1) it is, and 2)all the usual lumps and humps present in lesser gear are eliminated by the result of first class design and execution. The beauty of this is on the great gear, you can dirty it up to sound like that, but you can never clear the lumps in cheap stuff.
PS In the EDEN website, go to the METRO page in the products section. You'll see a prompt for a pdf of a METRO control panel with all the 'in-between' EQ frequencies printed on the panel. Helped me a lot.
J o e y
