So the conclusion is I shouldn't run single-ended headphone on balanced output amp by adding resistor, but running SE headphone on unbalanced speaker amp or balanced headphone on balanced speaker amp either directly or 'resisted' at 8 ohms are just okay?
Just to update a bit on some 'facts' when I run SE headphone on the box. If I used low value resistor for both L- & R-, I think the SQ about the same on both direct or resisted, But if i used higher value of resistor, say 5 times higher, the sound borked. In all cases it still produce hiss but very weak. I run my TP22 for more than an hour in this mode, but the heat from the alloy case cannot fried egg. May be next time I use thermometer to monitor the heat from the TK2050 IC.
Anyway, thanks for the advise