Abel
you will find that measuring the Impedance of your 'Speakers with only a multi-Meter can be difficult.
However an estimate might be achievable for the Low Frequency part of them by measuring the DC Resistance between the terminals of one box and from your reading then 'calculating' the nearest nominal Impedance.
For a DC Resistance of 5.5 to 6.5 Ohms you will likely have an 8 Ohm nominal box.
Likewise, 2.7 to 3.2 Ohms for a nominal 4 Ohm box.
This comes from the DC Resistance of your LF 'Driver itself, quoted by Manufacturers of the 'Speaker Chassis themselves as Re, and an additional small Resistance of the Crossover series Inductor - used in almost all Hi-Fi boxes.
It is not an 'exact science', so don't use the figures I've quoted as hard limits, but just as a guide.
As for a suitable Amplifier Power rating, this depends much on how hard you intend to drive your 'Speakers and their actual Power rating.
For Hi-Fi uses, often an Amplifier has an output Power rated higher that that of the 'Speakers and assumes that the available Power is available as Headroom to handle peaks in the signal. For Electronic Musical Instruments the converse applies.
This is down to the different Crest Factors of the signal you send to your 'Speakers.
Plenty more about this on the Internet with some searching.
hope this assists
Mik