same question for a vibrolux running two 8 ohmd spkrs so 4 ohms amp says not to run below 2 ohms -can i use the 8 ohm extn and the built in spkrs at the same time ?

For an amplifier that is safe to run loads down to 2Ω, you can safely connect two 8Ω speakers in parallel across it. What that means is you'll connect the (+) lines together and the (-) lines together. Both 8Ω speakers can be connected together this way, and that will present a 4Ω load to your amp.

If you were to connect another 8Ω speaker in parallel with two others - three 8Ω total, all in parallel - the resulting impedance is approximately 2.6Ω. That's still safe for an amplifier rated for 2Ω loads. But it's just barely above the safe limit so I wouldn't push it. Be cool with the volume knob.

Loudspeaker impedance isn't a fixed value, the number quoted is an average, usually rounded to the nearest 4Ω or 8Ω. The impedance is actually a fluxuating line that moves with frequency. The typical 10" driver, for example has minimum impedance at very low frequencies, peaking at a resonant frequency in the midbass, then dropping back down to minimum in the lower midrange. From there it rises gradually as frequency goes up. So the amplifier may have trouble driving the lower midrange frequencies, where impedance is at its lowest point. The trouble for you is that's where a lot of your signal content is.