I have used Mb450s with my SOundlab A1's with great success. They sounded great, and later I moved to the MB750s.
Unhappily my experience with the 300's was with the original MB300s without the signature transformers. The MB450s were much better in all areas I can remember. IMHO, the A3 will also appreciate the extra power of the MB450.
Unhappily my experience with the 300's was with the original MB300s without the signature transformers. The MB450s were much better in all areas I can remember. IMHO, the A3 will also appreciate the extra power of the MB450.