Algirdas,
You could (and would) but maintaining the bank angle proves that Whopity's explanation is false.. Therefore, bank angle-constant, power-constant, speed constant. The only thing left to control the speed, which would be reducing if you maintain altitude, is adjust the attitude.
Whopity,
Attitude+power=performance is an old cliche. Please supply us with the correct mathematical equation showing the inverse proportionality.
In your wriggling you've just just introduced a fixed power setting where no fixed power setting belongs. The required constants for an approach are airspeed and flightpath. Manifold pressure or torque will change due to atmospherics and the required power because of the wind. You can maintain the glidepath with the stick, just as you would a constant height, but I reckon any more than a needle-width, as I said earlier, is more likely a trend or change in conditions requiring a power change.
If you want to discuss which one should do first then we'll have to come right back to perhaps one knot above the stall and apply full power, whilst holding everything else constant (and at a safe height). It's a good technique to get a clean entry into a spin or flick!