Thanks for reaching out via the Bose Community. Welcome to the site!
The SA5 Amplifier is not designed for use as a 2.1 setup. Ideally, you should use either 1 or 2 pairs of speakers for the Speakers A, and Speakers B outputs.
I would recommend cross-referencing the below SA5 specifications with your speaker to confirm compatibility.
• Output Connections: Two pairs of speaker outputs (1 pair for Speakers A; 1 pair for Speakers B)
• Power Output: 100 watts per channel, nominal 8 ohms per pair
• Use only unpowered speakers. Do NOT connect any make or model of powered speakers. Doing so may cause damage to the equipment.
• One pair of speakers requires 4-ohm minimum each.
• Two pairs of speakers require 8-ohm minimum each.
If you require a user manual for the SA5, you can find one here.
I hope this helps, and if anyone else has achieved a similar setup, please feel free to add to the conversation.