Everyone is certainly entitled to their opinion, but any time there is an issue with the bass, subwoofer recommendations seem to be plentiful regardless of the scenario. Looking at basic specifications of the two speakers in question,
B&W Matrix 802 Series 3
Frequency Response - 42Hz – 20kHz ± 3dB
Sensitivity - 90dB spl(2.83V 1m)
Impedance - 8 ohms nominal (minimum 3.4 ohms)
Vandersteen 3A Signature
Frequency Response - 26Hz – 30kHz +/- 3dB
Sensitivity -
87 dB, 1 meter/2.83 volt input
Impedance -
6 ohms nominal 4 ohms minimum
Since the 3A's are 6 ohm that makes their 1 watt sensitivity ~85.5db compared to the 1 watt sensitivity of the 802's being 90db. With the 3A's going deeper in the bass but with a much lower sensitivity, I'm thinking the issue is power, but can only guess not knowing the amp being used. Also, having experience with both speakers, I do somewhat agree with this comment,
"Both speakers play well with the right set-up and equipment. If not, the Vandersteens may bore and the B&W’s may glare/sizzle. I can't imagine them working interchangeably in the same system."