How strict is your definition of "lightweight"?
In other words, most amps (Aguilar, Mesa, Genzler and many of the other amps popular here) in that power range are roughly 5 – 6 lbs.
Would you automatically eliminate those in favor of a reliable amp that only weighs 4lbs?
If so, the Quilter Bass Block is your answer.
Again - sounds like the previous version of the Quilter Bass Block - the 800, is the one for you.
Fewer knobs than the 803, and it has an excellent reputation here.
What cabinet are you using?
Can we assume your overall goal (head & cab) is the "biggest bang" in the smallest lightest package?