The UPA-2 cannot be bridged, so I do not believe you would want it. However, the UPA-1 is a mono amp by design, so would fit your needs at the same cost.
You will probably get more answers from those familiar with one or both (I have neither). However, since you already have A500's and I assume are comfortable with any concerns over fan noise and aesthetics (which is not a concern for a true Audioholic), on paper, the A500 puts out more power than the UPA-1 and will stack perfectly with your other A500's while leaving more money in your pocket.