Can't drive Proac Studio 110 with low budget amps

The difference between a 50 wpc amp and a 100 wpc amp is 3 db. Since the ProAc's are not really efficient speakers, I'd recommend going with at least 100 wpc. 50 wpc may cut it in a small room though. The bigger the room, the more amp power is required.
If you say the woofer is barely moving when it should be rocking, I'm led to wonder whether your AV receiver has some low frequency crossover filter that is rolling off the lows for a subwoofer. A real preamp/amp setup should eliminate that issue. Hopefully there is no damage to the drivers themselves.

Jmcgrogan2 is correct about the mid-fi receiver. Not enough current. Dynaudios are oft difficult to drive well. A Rega Brio R (50wpc) will drive them decently or a Maia lll will really kick them around. They also really like tubes such as the Manley Stingray (50wpc Class A).

With a lot of speakers it is not so much the raw power available, like for example 200wts at 8 ohms, as it is the current delivery to speakers with lower impedences i.e. 4 ohms and below. Amps which double power output at 8 ohms when it droops to 4 ohms or lower, are a good candidate for success. Those which do not may well have difficulty driving your Proac's I suspect (I've never seen the actual impedence curve of your speakers).

Re raw power - 50wt amps which double from 8 to 4 ohms will sound more powerful than their actual wattage suggests especially if your speakers have an efficiency of 90+db. If you don't listen to them excessively loud in a medium size room you could probably drive speakers with a 86db efficiency. I've used an loved high current 50wt amps with 86 db speakers with difficult impedence curves in a medium size room.

FWIW, I've never had Proac's in my home.

Thats what I am going to do: find a powerful amp for a test. One thing that confuses me a lot is the fact that several people told me their 50 watt amps can drive Proacs pretty well (Audiolab,Naim,Arcam). Also, placement is not an issue here, I can see that the woofer is barely moving when it is supposed to really do a hard job.

First I'd try moving the speakers closer to walls and reflecting surfaces, which may re-inforce bass. Secondly, these speakers deserve better amplification than some mid-fi receiver. Try a seperate power amp. You should be able to get by with at least 100 wpc. Remember, a 100 wpc amp will sound very different than a 100 wpc 7X100 AV receiver. My 100 wpc amp weighs in at 85 pounds, it has a much stiffer power supply than any receiver, and does not run out of steam when pushed.
200 wpc may do even better for the ProAcs.