Ahhh--Problem solved. Adding a REL sub-bass unit...

(I always enjoy hearing what engineers hear)

Almost no one ever will. The room and electronics are vastly different. Nowadays, by the time the label gets done ’improving’, any similarity between what was mixed and what’s on the disk is quite small. Doubly so for ’remasters’ See http://ielogical.com/Audio/#ReIssues

A system without the bottom octave is a pale simulacrum and loses more that ½ the immediacy of a live performance.

IMO, most systems with subs are egregiously awful. It’s often not the fault of the sub, but the set up:
1. 0-180° Phase controls are only valid at one frequency. If the phase is not correct, the system is likely better w/o the sub. Filters change the phase response making the two controls interactive. Given that small subs have large EQ curves built into their amplifiers, the phase pot marking is next to worthless in terms of absolute phase. 180° phase switches are next useless except when used in concert with a 0-180° phase control.
2. A system with powered sub should use a XOver to remove the lowest frequencies from the mains and is preferrable ot trying to ’blend’. Doing so reduces main power requirements, driver excursion and distortion. Feeding the sub first and then the sub feeding the mains is bollox. A simple single capacitor will give a 6db/octave filter which will give a nice blend with a 18db/octave sub rolloff, provided the sub phase is inverted. Getting the phase and frequency correct is tedious. It can be calculated if one knows the amp EQ, driver response, crossover type and slope. Having a close starting point is vastly preferrable to most of the ’instructions’
3. Many systems crossover an octave too low. IMO, speakers should be crossover an octave above where they start to fail badly. In the case of small monitors like LS3/5a, Spica TC-50, that’s around 100Hz. The problem is then to find a sub with similar sonic characteristics to the mains. If the mains are capable of true 30Hz, IMO there is almost no need and even less probablity that adding subs will improve the system unless the mains are placed for WAF rather than sonics.
4. Ported subs? Never!

When properly set up, pop bass is focused and does not stroll. Tympani and bass drum become focused. The sense that one is there is out all proportion to the measured delta.
See http://ielogical.com/Audio/SubTerrBlues.php for a bit of a primer on integrating a small sub and small monitors.

@steakster 

Disinformation abounds here.

Prefer it if you can refute engineering facts. 

2. - Loudspeaker designers spend great effort creating crossovers for LF/Mid/HF bands. Mathematically it's almost impossible to mate a 12" driver to 8" or smaller driver in a vastly different box with no filter and get seamless integration. The level may be relatively flat, but the phase will be a nightmare. Until one has heard a minimum phase error system playing all 10 octaves, one has never heard bass correctly reproduced.

4. - Ported speakers have large amounts of time / phase shift, and the lower you go, the worse it gets.: "Bass reflex cabinets have relatively poor transient response, causing "smearing" or a longer resonance of the bass notes. Though the sound coming out of the port may have the same phase of that from the front surface, but it can never be at the same time, thus, the extended bass energy is really noise disguised as signal. The disguise works only when the sound is a continuous tone (one of the reason why some people prefer some particular kind of music for their audio system), but reveals itself most apparently at reproducing percussion sound."  https://en.wikipedia.org/wiki/Bass_reflex#Limitations 

As a great musician friend once said on hearing my system "Those little speakers aren't putting out all that gorgeous bass?!?!?!?!?" Informed of the sub, he stated "Every other subwoofer I've ever heard just boomed!!"

You took the words right out of my mouth. So much opinion spoken as fact in @ieales post. And most of it wrong.

Again, please refute with engineering facts. My focus is on music reproduction, not HT. Ported subs can go lower at the expense of linearity, phase and transient response.

I think many fail at integration because they have their sub(s) crossed over too high and with far too much gain.

Most program has no real bass. By crossing over low, almost no signal is reproduced by the sub.

Again, IMO, not enough attention is paid to phase. 80Hz is 14 feet. 90° phase error is ~3ms. Humans use the time delay between their ears for directional information. Phase error causes instruments to stroll and fatigues the listener.

In a concert hall, tympani, bass drum, bass, etc. are all localizable blindfolded and they do not stroll. In the studio, on phase coherent monitors, kick drum, electric bass, either DI or mic'd, don't stroll either.

Too many HiFi systems have horrendous low end phase which causes the aforementioned instruments to stroll or be impossible to localize on well recorded program. A poorly integrated subwoofer is a headache in waiting.

For more than 4 decades, minimum phase error has been a primary focus. For the same period, listeners always comment lifelike, accurate, precise, etc., regardless of room and hardware.

How many have bothered to calculate the phase response of their XOver, loudspeakers and subwoofer amp to integrate their sub and then measure and analyze the result to tweak and verify?

I said "I like 1st order high pass on the main amplifier" - that means that you roll the low end out of the mains.

A single capacitor in series with the amp input works beautifully.

I was a recording engineer. I produce live concerts. I’ve taken RTAs to venues for decades.

There is very, very little sound below 60Hz. A double bass is ≈41Hz on open E. A bass drum after ring can have some 20Hz-ish sound, but only in some halls. Piano can go ≈27Hz, but it’s hardly ever played that low. Few studio recordings have any real low end.

IMO, the point of a sub it to reduce the load on the main woofer. When the mains start heading south in an anechoic chamber, phase response and impedance are starting to go south as well. Crossover an octave higher and there is a LOT more power and control available for those frequencies because it is not being eaten up trying to drive the woofer.

Tube amps in particular benefit as transformers are not particularly good at low frequencies.

I like 1st order high pass on the main amplifier and 3rd order low pass on the subs. That puts the phase correct-ish at XOver.

If you can afford it and have the space, use two subs very close to the mains and GET THE PHASE RIGHT!!!! A 0/180° control is mostly decoration. At the very least, a continuous 0 through 180 control. It’s easy to add a ± phase switch to most subs with a DPDT switch between the driver and the internal amp.

The difference good sub integration makes is out of all proportion to the frequencies.

P.S. Martin Logan subs have tremendous flexibility and are very easy to integrate. I looked at all manufacturers earlier this year and ML offers the most bang for the buck. I'd love a pair of top of the line JL or REL, and while I could afford it, I just can't justify the cost. ML integration beats them six ways to Sunday with multi slope XO, ARC and remote control.

RELs don't do it this way, they are designed to pick up where the mains drop off.

That, my friend, is nonsense.

If the sub has controls, the user chooses where to integrate. No two rooms or systems are identical. Move a complete system from one room to another and it's a whole new ball game.

Most of the time it is done terribly.

It's important to get a sub that has a similar 'signature' to the mains. Tools like REW help immensely.

What do you recommend for a single capacitor that is "plug N’ Play"?

This image shows the circuit topology of hi pass

f = 1 / (2πRC)

f in Hz
R in Ohms of amplifier input
C in Farads of series cap

C = 1 / (2πfR)

This calculator will do the math. Scroll down to the calculator. Select Calculate Capacitance, enter amp input R & corner frequency. Calculation is pF. Best dielectrics are Teflon, polystyrene, polypropylene in that order and descending cost.

Parallel if you want to exact frequency, but ±1nF is close enough for most amps except those with very high input impedance. If you want to go nuts get an LCR meter and match as tight as you like, but realize the speakers are not that close, especially at the bottom!

This image from http://www.ielogical.com/Audio/#SmallestThings shows how to assemble.

You feed the full range to the sub, using its controls to roll out the highs and feed the filtered side to the mains.

When connecting for first time, disconnect mains from amplifier and turn subs all the way down. Slowly bring up subs with program playing. Should be just low end. Turn off program. Measure amp Output voltage. It should be nearly 0mV.

PM any questions.

Ouch! on so many levels.

Low end is always the poor stepchild as it conflicts with parameters necessary for higher range of driver.

Phase controls adjust at single frequency. See http://ielogical.com/assets/SubTerrBlues/PhaseControl.png at http://ielogical.com/Audio/SubTerrBlues.php/

Smaller sub than main driver? Really!
Different sub models? Really!

Testing CD and no instrumentation? REALLY!

Easy == compromised.

I'd have to hear it to be convinced.