How is the DB PVAB typically calculated when attempting to put a value on the DB benefit when the plan does not offer lump sums? The couple is just attempting to ascertain the value of their estate at this time.

I've done this a few times. I use the PV assumptions in the plan, as they would apply for purposes other than lump sums (assuming the plan has such a distinction). Other reasonable methods might be a current liability calculation, or an ABO. However, I prefer not to use an interest rate that is variable. Others may have the opposite opinion.

In all cases, any response to this question will be in writing and very strong caveats are important.

They should tell you what assumptions to use. Alternatively, giving them a set of numbers based on different rates will allow them to see the range of possibilities.

None of the "official" rates used by the actuary will have any meaning to their situation unless it is owners of a business with a very small plan whereby the value may have more meaning beyond the benefit itself.