I am not sure I buy your number of speaker argument. In my real life experience an L2 speaker (my current amp rig with HD500) produces more sound volume than a 60 watt tube amp with a 4 X12 or 2 X12 speaker cabinet.
Are you running this for guitar? If so, then it's an apples-and-oranges situation---skip to the next section of this post.
Most of the loudness for bass depends on the efficiency of the speaker cabinet design. Perhaps your existing cabinets are inefficient. A 60w bass amp seems rather small given the typical efficiency of many commercial bass cabinets. The 1x12 DIY cabs I built are very efficient, producing more usable loudness than the TC Electronics 112 cabs they replaced. [Disclaimer: They aren't my design, so I'm not taking credit for that. They are, however, very loud, light and easy to build.] Commercial manufacturers use 3/4" plywood because it's cheaper than building a lighter cab from 1/2" ply with internal bracing. They only lift it once when it goes onto the truck---you lift it at every gig!
Get an L2 or L3 as the 'top' and get an L3s (sub) for the low end.
This depends on where the sub is crossed over. Most bass guitar is way above subwoofer frequencies. Sure, you get some boom from a sub, but that isn't what you want onstage. (I'm not talking about a large house PA system, where the subs can be properly located and controlled.) Most of us tend to play varied venues with acoustics ranging from decent to downright miserable. Having a sub onstage is often a recipe for disaster.
Remember that a 4-string electric bass has a lowest fundamental at about 41Hz; a 5-string will go down to 30Hz. Much of the sound of electric bass is in the harmonics, of which almost all are well above the subwoofer zone.
And, since one of the original objectives was size and weight, I think that adding the 85lb L3s sub to a 40- or 57-pound L2 or L3 trends to make the situation worse.
Seems to me that changing to a pair of 210 or 212 lightweight bass cabs is a better solution. They would be voiced appropriately for bass guitar and would have adequate xmax/xlim to deal with the heavy cone excursion required for bass.