Impedance?

I'm thinking about adding a pair of subs each with 2 JBL 18" speakers. You can get the JBL's in either 8 ohm which would give a net impedance of 4 ohms per side, or 16 ohms which would give a net impedance of 8 ohms per side. What are the advantages/disadvantages of either option?

Tell us about the amplifiers?

The amp driving the subs is an Onkyo M504 165WPC into 8 ohms.

I just looked at the specs of your amp and all ratings are given using an 8 ohm impedance and there is no power output spec for a 4 ohm load which is telling. Given that, I would stick with the 8 ohm version of the subs you are looking at.

Tell us more about those JBL subwoofers; power requirement, etc.

https://i.ebayimg.com/images/g/J0EAA...vX/s-l1600.jpg

The Onkyo M-504 stereo amp is a vintage beauty.
If using System 1 or 2 it will drive 4 Ohms speakers.

It might be a good idea to put a fan on top, those flat types that are efficient and quiet; over the warmest spot. Anyway 250 watts into 4 Ohm loads should be avail in the Onkyo M-504 stereo amp.

But like I said, I would check the power requirement of those big JBL subs first.
And that M-504 is simply gorgeous, one of the best looking and proud of it.
You know because you own it, and you also know how it looks inside as well.
The rating is 1 kVA for the transformers (power consumption), so it's not a kids toy here, it has serious juice (current) and enough microfarads total to sing with aplomb.

Sure, it's not a $100,000 amp, it's a $2,000 one (slightly below, MSRP) ... circa mid-80s (1987).

Yes I'm very happy with my pair of 504's and my pair of M5030's for the mid and tweeters. The JBL 18's are rated for 600w at 8 ohms

A pair of M-504, even better.

The M-504 pair will drive the JBL subs.

Attachment 28528Attachment 28529

* Bonus:
https://www.youtube.com/watch?v=il-kG21eMsQ

Question: You'd be using the subs in a stereo hi-fi system (music), or in a multichannel system (movies)?

* Ask JBL's sub manufacturer (customer service) the same question you asked in your first original post. ...The subs recommended (impedance) for the Onkyo M-504 pair of amps.

The subs are for a stereo and one 504 will drive the subs, the other the 12" JBL mid-bass speakers

Ever thought of getting powered subs, with their own internal plate amps?

Your M-504 should drive the JBL subs. Ask JBL if they refund money just in case your Onkyo amp is running hot. Usually sub manufacturers offer a 30-day "Grace period" with full money back no question ask...just pay the shipping. If the shipping is not in your plan, ask the manufacturer first on their recommended external amps, and mention your M-504 ... they know best, better than most of us here.

Another thing; 18" drivers for music listening is quite big. Ever considered 12" drivers, or multiple 8 or 10" ones. Do you listen to synthesized music (electronica) a high volume level, and organ music quite a lot?

Are you into classical orchestral, big jazz bands, motion picture soundtracks?

The M504 has been driving my VMPS Large subs well for years. I like bass I can feel. I enjoy synthesized music, and hard rock, maybe classical. Just about anything sounds good on a good system

Then get the JBL subs (8 Ohm).

Disadventage: None
Advantage: Perfect

* Ask JBL's technical department if the 16 Ohm gives you an advantage or disadvantage over the 8 Ohm, and matching them with your Onkyo M-504.
My best guess: None ... 8 Ohm is fine.

Do you mean a pair of 16 ohm wired in parallel for a subwoofer system impedance of 8 ohms per side?

No, the other option; a pair of 8 Ohm for a system giving you 4 Ohm in parallel.

Got it and thanks!