Driving 8 speakers outside

[the DRIZZLE]

This. 8 speakers is actually perfect. For each channel, wire a pair of speakers in series, then another pair in series, then plug both pairs into the receiver's output in parallel. Do the same for the other channel. The load on the receiver will be exactly the same as if you only had one stereo pair. Total volume produced by the receiver will be exactly the same as if you only had one stereo pair, it's just that each individual speaker will have 1/4 the work to do to produce that volume. And 135 watts/channel should produce plenty of volume.

Isn't the impedance different?

 

[Viperoni]

Nope, the impedance would be 8ohms per set of 4 speakers.

It's always better to get more cone area than more power when trying to fill a large area.

Going from 1 speaker to 4 while keeping the same impedance load (and thus the same power input) gives you a 6db gain.
You'd have to give 1 speaker 4 watts instead of 1 watt, or 400 watts instead of 100 watts to gain the same increase in output.

A good choice for $250: http://www.parts-express.com/pe/showdetl.cfm?Partnumber=248-749
Sounds good and is decent quality too. Expect power output to be on the low side of spec, but it will make the rated power. FWIW I have a Behringer EP2500 (2500 watts RMS bridged) and it does bring home the bacon.