8 ohm systems need to be wired so the load on the amp is 8 ohms. That means you can use 1 speaker, or 4 speakers, or 16 speakers.

The four speaker setup is speakers A and B in series to make 16 ohms; speakers C and D in series to make 16 ohms; those two combinations in parallel with one another to make 8 ohms. And, since four wired this way are 8 ohms, four such groups wired similarly with one another is also 8 ohms. It's called series-parallel, but that term sounds WAY more flexible than it really is. A slight bit of flexibility can be gotten by wiring for four ohms.

But 70 volt speakers? A recent restaurant we did has one speaker in each bathroom and another in the office (A, B, C). The main eating area is not too large and three speakers are perfect for that (D, E, F). The rest of the place is fed by four speakers (G, H, I, J).

Here's where it gets tricky. We want a volume control on the single speaker in the office. So we put one in. Another VC on the pair in the bathrooms. Another on the eating area... and that area has two ceiling heights, so one of the three speakers must be louder than the other two to make the volume the same across the whole area. The rest of the place is on yet one more volume control.

With 70 volt, this is a piece of cake. Selecting a different tap for D makes it louder than E and F but it still goes up and down in volume with them. The office can turn the sound off if they need silence. All of this is impossible with 8 ohm wiring setup.

Better yet, if it turns out that a little fill is needed somewhere, one more speaker can be added. Try doing that with an 8 ohm setup!

If the issue is that 70 volt costs more, learn what products are good and take a chance on buying on ebay. Restaurants are forever being built and going out of business and perfectly good product is available... if you know what to look for. You can tell your client that you're saving them a ton of money if they will accept that the product is not new, and they will probably go for it.