This is requesting a quick answer(I know it is late in the year), for a situation where there is:

1)W2 income with a 401k that gets contributed maximally with matching by employer

2)K1 income that is subject to SE tax from an LLC partnership due to a consulting agreement...equal partners only. No employees.

3) 1099 income outside the LLC

All for the same person...

Want to know if the 1099 and K1 incomes can be pooled to make a larger DB plan contribution. This is the first year we will be doing a DB plan

Start with this reasoning.

You only count income from the adopting employers, of which you appear to have three.

The sole prop gets 1099 income.
The 50% partner works for a partnership.
The W-2 wages are working for another entity, I assume.

If the partnership income and the 1099 income are used for plan purposes, both organizations must have
adopted the plan. For a DB plan, then you must include both partners in the plan to avoid 401a26 issues.

You did not disclose any info for ASG issues or CG issues, such as common control, management services,
service organization, etc. So I don't know what to do with the W-2 wages, nor the other employees of the
other entity.