Subwoofer with Floorstanding Speakers?

I have the Vandersteen Sub 3. Costs more than the others, but integrates seamlessly and frees up your amp from the lower frequencies.

B

Like; "Slaw" above. I use the Martin Logan "Depth" in my small system.

    But use (4) of the Martin Logan, "Descent" subwoofers in my main rig.

Which is an actual, "Line Source", "Dipole" system.

Just (1) of the "Depth" subs will impress you. Especially for the amount of air it will move, and that air at very impressive speeds. And the accuracy still floors me. Nothing comes close to it at that size.

But having (4) of the "Descent" model subs integrated into a rig?

It can be beyond words...

The key is the "Integration".

 I use a quality, "The best I could find" active crossover. The sub's use "Blue Truth, Air", "Balanced XLR" cables from the crossover to the subwoofer's amplification section. And then seamlessly integrated with, "Linkwitz-Riley", Constant Voltage, 4^th Order, "24dB/Octave Slope".

Beyond Words...

2 floorstanders 2 subs here. Can't do without the subs now, was a great investment. 

Rarely, if ever, is the location of the main speaker optimized for BOTH bass and mids and high frequencies. It's the reality of acoustics within a room. Furthermore, I have never heard of someone removing their subs IF they have been properly integrated and dialed in.  

Not only will your Def Leppard sound better but so will every other type of musical recording.

First step would be to upgrade power amp. Then the rest of the audio chain.

Adding subs is a complicated matter and a final step, if needed.