It is possible that there is a bad driver chip on the Xylotex card and when the new power supply starts up it is going into overcurrent and shuts itself down.
Pull the Xylotex card and send it back to Jeff at Xylotex and offer to pay for a repair. It will be less than buying a new one.
Before you hook up the repaired Xylotex buy/borrow a DVM (digital voltmeter) and measure the DC volts of you new power supply. Carefully observe the DC polarity and make sure it gets connected correctly to the Xylotex.
Before you hook the motors back up check all of your wiring and use the Ohmmeter to check for open or shorted connections to the motors. Check each winding against the motor frame for shorts. Throw away any motor that shows a frame to winding short. Make sure the Xylotex card has adequite cooling with plenty of airflow across the heatsinks.
Never plug or unplug a motor with power on.
Never let two motor wires short with power on
Never let a motor wire come loose with power on.
Never use more than 28VDC motor power.
Never hook up power backwards to the card
Always set the motor current adjustments per the instructions
You can run a card with no motors attached or just one and move it (with the power off!) to each axis and test that axis.
Power supplies do sometimes fail. Normally a switching type supply will not fail and put out more voltage than its rating. Only if the feedback loop gets compromised (in the supply) could it put out more than it's rated voltage.
Dan, we have sold over 75 Xylotex based mutilaxis controller boxes in the last 18 months. None have come back defective. We use an external 24V 5.5A commercial switching power supply and we have had to replace one that failed with no output.
The Xylotex when used properly within it's specs and temp range is very reliable. I have one in a proto of the 4 axis product we sell that we use on our table top router and it sees hours of use every day cutting out front and back panels, aluminum sub-plates and doing other jobs. It often gets left on overnight (grrrr!) and it has never failed or had to be taken out of service. Still going strong after 2 years.