Talk subs twin vs single

[L18M]

Anyone here got any experience with subs?

I\'ve just installed a Pioneer TS W77 into my car and its not as loud as i\'d imagined. The OEM speakers had a fair amout of bass so this was only a very small improvement. The reason for this Pioneer is due to lack of space (cabriolet).

If i were to install another one if these. What would the result be?
Louder? more kick? more noticeable running twin over single?

Lastly, does it make a big difference if the amp is wired in using RCA or hi level from OEM speakers?

Any feedback muchly appreciated :cheers