I think its current as a large section of the impedance curve is closer to 4ohm than 8 ohm.Current is not the issue here. If the amp can make the power, the current is present. This is because power cannot exist without current. The power formula is
Power = Current x Voltage.
Now the idea that the speaker is 'voltage driven' is verbal shorthand for the fact that the designer of the speaker expects the amp to behave as a voltage source (otherwise amps would not be called 'power amps'...). The amp does not need to double power into half the impedance to be a proper voltage source!! If the amp has sufficient negative feedback (and it does) the amp will simply be limited to 60 watts in the 4 ohm portions, and the higher impedances will see less power; half of that if double the impedance.
It may simply be that you don't like how your amp sounds on this speaker, but the idea that it does not drive the speaker correctly is simply not true.
IOW your amp has plenty of current to do the job. The current is not the problem!
In that regard, a class A amplifier is a good suggestion, not because it might be able to make any more power or have more 'current'; it might simply be that it sounds better. Many (cheaper) class D amps don't do so well in the bass as far as sonics go, while all the time having the specs needed to drive the speaker!
If you are on a budget and 30-40 watts is enough power, you might want to try a tube amp on this speaker and see what you think. A Dynaco ST-70 should do the job and they are not particularly pricey. You may find that tube amps make better bass than you thought- I've seen many that put many class D amps to shame.