It depends which area's of the sound you would like to improve on first.
I use a sub with my speakers and have done with every pair ive owned except some big PMC IB1's. I couldn't afford a sub big enough to underpin them
When looking for a sub, be aware of how low the upper frequency cut off can be set. An active sub has a frequency dial, sometimes adjustable from 30hz to 120hz, 40hz to 120hz or sometimes lower like 16hz to 99hz.
Im on my 3rd active sub(Rel Strata 5), its frequency can be set between 16hz and 99hz, so depending on your room and how big/small your speakers are, will depend on how you set it up.
My previous sub's were a monitor audio ASW100(lowest frequency position of 40hz)
And a Rel Q201e(lowest frequency position of 26hz)
I always found that, even using standmount speakers, you couldn't set the frequency of the sub low enough to intergrate with the speakers properly due to the filtering and 12db per octave crossovers used in the sub's.
They always play frequencies way above the supposed limit you have set the dial to.
You end up having to turn the sub's volume down so low to intergrate that it doesn't produce the lowest frequencies properly.
If your using the sub for home cinema purposes, this isn't an issue as you usually set the sub up at the suggested 80hz to make the most of big action movies.
But with music, you want it to be subtle. The sub should not produce bass that the speakers already produce. Bass should not sound louder with the sub on. The music should sound exactly the same, except when really deep bass is called for it will be produced as if by the speakers when setup correctly.
A test cd with frequency tones is vital to set up correctly and some people find a sound level meter useful.
With my previous speakers(pmc db1i), i had my sub set at 20hz
with my ATC scm19's, i have the sub set at 16hz to intergrate properly. You never hear the sub as such, but play music with deep bass, then turn the sub off and play again and you will notice whats missing.