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.

I've recently become a dealer for VMPS after having used their speakers for the last decade. Their passive subs are extremely musical and powerful not to mention bargains for the money. They come pre built or can be ordered as kits. Check out the reviews on audioreview.com and also check out the Vmps forum at audiocircle.com. Also I thoroughly understand why you'd want a passive sub as it can be much more versatile and you can use a much better amp than a plate amp.

Another option is to build your own. On the madisound and partsexpress boards there's alot of knowledgeable people that are very helpful. A sub is relatively simple compared to a multiway speaker and is fairly easy to build.

Not too many speakers in my opinion can do a truly thorough job without at least one sub. I run my speakers which are full range(Vmps RM 40's) with a sub. The Rm 40's are running full range and I've got the sub crossed over at about 45hz. Others do the same with similarly powerful speakers and there's a thread here with some info on that. Then there's people who don't care for bass as much as many of us. To me the bottom octave must be produced accurately and this means it must be well articulated, have power and weight. Best of luck to you.