Is this the older model ha540 or the ha540 mkii?

The MK2 is the new one I would bet that all are the MK2 version, but that would be up to the individual seller. Looks like Amazon is not very good at updating the listing.

Is this amp for operation on 110 to 240 Volts ?

This item comes with a universal voltage AC adapter. It has input voltage of 100-240v 50/60hz.

What's the output power of this? How many milliwatts or watts?

Max. Output: 500 mW/side, 1 W/Stereo ( @ 16 Ohms )

Is this pure tube, with no transistors in the audio path?Thank you in advance!

I forgot to mention, this a great sounding amplifier. I would highly recommend changing the tube. I changed the original tube with a Mullard tube but am now using it with a JJ ECC803 / 12AX7 Long Plate Gold Pin Vacuum Tube and it sounds great. The tube does make a difference.