Need advice on a passive subwoofer

Why do you want passive? It only makes things more complex.
Why do you want 4ohms? That's usually disadvantageous since it will parallel what you now have.

Kal

On the advice of someone who knows my setup (a TAD Cayin TA-30 tube integrated), using a passive isn't that complicated at all: "first connect to 0 & 4, then connect the high driver '-' to the 4 tap, '+' to the 8 tap (double stuffing the 4 tap), using the entire output transformer winding in a semi-isolated manner. Its like having 2 amps in one". Or so I'm told. I've found that there is more than enough current (I used to drive Legacy Classics w/no problem) as long as the speakers are effficient. The speakers that I will be getting are 93db efficient. All I may need is a modest, musically accurate sub.

Have to agree with Kr4. Too much of a pill with a passive.

If the sub is too loud or too soft how will you integrate it - put a resistor in line - Yuck.

If you're that serious about it, look at some of the REL's that do a good job at line level, and you can adject whatever you need win its performance.