M2x or M2 - A tighter fit to the unit will make for a heavier BASS experience, please tighten your straps!
S2 - Try a firmer, stiffer chair that won't absorb the low frequency energy. Soft couches and padded chairs don't do a good job of reflecting the energy into your body. A good body, chair and placement combo can greatly improve the SUBPAC experience.
A few more possibilities to consider:
Low input level:
The SUBPAC needs a healthy level to work well. For most people ipod at 60% and SUBPAC at 40% will make their noses itch and eyeballs shake.
If you want to listen quietly and feel lots of bass (and you have it cranked and its still not enough) you will need to use a separate headphone output from your interface. This output will contain the same audio just at a different level. Now you can feed more level into the SUBPAC and feel it more.
Some headphones have a volume control built in. This will allow you to send a fuller signal to the SUBPAC and still monitor lower in your comfort level.
Product life note: If you are at maximum volume and max intensity and the product begins to get warm or hot, take a break and let it cool off. Symptoms of a blown SUBPAC include heat and reduced vibration after a long period of heavy listening. Excessive weight or forcibly leaning on it during these periods can create more heat and intensify the problem.
Still need more bass?
Try a firmer, stiffer chair that won't absorb the low frequency energy. Soft couches and padded chairs don't do a good job of reflecting the energy into your body. A good body, chair and placement combo can greatly improve the SUBPAC experience.
Still need more?
Some forms of music (classical, opera, some jazz, etc..) have very little bass by design. These will feel weak on the SUBPAC if you are expecting an eyeball rattling experience.