can we say 'the big speaker systems like proac d40 or b&w (4ohm) generally don't go well with tube amps'?
Yes, and I can take it a step further- 4 ohms does not go well with *any* amplifier, if **sound quality** is your goal. If *sound pressure* is your goal you will get some small benefit from using 4 ohms with transistors, but even transistors sound better into 8 or 16 ohms.
IOW if you are a loudspeaker manufacturer this is a simple way to get your speaker to seem smoother and more transparent- design for 8 or 16 ohms instead of 4.