It seems that adding another piece of equipment to the audio chain would degrade the sound quality to the main speakers (air, immediacy, the mid range, etc). Or am I way off here??
Currently have a preamp with no sub-out and low output tube amp. Plan to use line level to the sub and then out to the amp. That way speakers would not play full range (above 80hz).
Tghooper
What might you lose?
You might lose your mind if you have too much trouble blending the sub with your mains and the sub doesn't have the controls you need to do the blend. Otherwise, adding a sub or two can be very rewarding, especially with a system based on a low-powered tube amp.
There is a lot of advice on this thread on how to do it, and I think most contributors are advocating for what worked for *them*, but it may not be the best for *you*. I've blended seven different sub configurations in four different systems and they've all succeeded but my methodologies have varied according to several variables:
--The room acoustics
--The bass extension of the main or satellite speakers. on full range I cross over at 40 Hz; on some small satellites you may have to cross over at 70/80 or even 120 Hz.
--The I/Os on the sub--single LFE line level, stereo line level, stereo line level I/O, speaker level, speaker level I/O
--How much phase control the sub has; some have none, many have just a 0-180deg. switch, some have a 3- or 4-way switch (0, 45, 90, 180 deg.) or continuously variable (my latest have 0-360 continuous)
--Crossover control--hopefully continuously variable from around 40 Hz up to 80, 150, maybe 200
Conventional wisdom says that you get more dynamic range and more amplifier efficiency by using a high pass filter to the stereo pair (mains). No doubt this can be true in some cases with careful crossover matching and placement and/or phase setting, but in my own experience I get a better blend by running the mains at full range and blending in the subwoofer at the lowest possible crossover point. If the crossover point is too high it starts making vocals and some lower midrange/upper bass sound thick and murky.
This also flies in the face of conventional wisdom, but I've had a couple of subs that actually sounded better using the speaker connections than the RCAs. Don't know why; on these certain subs (2004-era Mirage LF-100 and LF-150) the subs sounded faster, fuller, more musical and better level-matched with the speaker cables than with the RCA connection.
A month ago I added a single tiny Mirage MM8 sub to my 2-channel stereo with Mirage OMD-15 tower speakers. The MM8 has no speaker level inputs and no outputs at all, so there's no option to sent a high pass signal to the mains. The towers have high handling capacity and are rated down to about 36 Hz, but I wasn't really getting that in-room. The sub, crossed over at about 50 Hz, helped deepen bass extension and kept the sub from murking up the upper bass handled by the towers.
These subs are a good match for the speakers. The subs are sealed, very powerful (1200 watts peak each), and extremely quick, so they easily match up with the mains. As 9" cubes they're made to blend with music systems or add bass to small HT systems, but not to rattle your teeth like a JL Gotham.
Another deal came my way and just yesterday I added a second MM8 to this system. It's just way better with two. The second bass smooths out the bass response (less peaky) and it adds about 6 dB dynamic range.
The funny thing is that somehow the mains (even though I'm *not* running them with a high-pass filter) sound more spacious, more dynamic, more 3-dimensional, more clear.
Right now I'm playing solo acoustic guitar and somehow the sound is simply opened up. Not overly bassy, not murky, but rather airy, more real, more dimensional, more fully fleshed out throughout. I'm a believer now.
But also, where this was previously a somewhat modest system best suited to semi-acoustic pop, rock, small group jazz, vocalists, and chamber music, it's now ready to boogie with big band and full-scale orchestral pieces such as Pictures at an Exhibition, Pines of Rome, and The Planets.
Those who have far bigger budgets who have implemented subs that are linear down to 14 Hz or so testify that the infrasonic information energizes the listening room more like what you sense and feel in a live concert.
I'm well on my way to thinking that if you want a truly fully convincing system that draws you into whatever kind of music you play on it, it could start with the subwoofer(s). In other words, it may actually be beneficial to allocate more money for the subs than the mains. I have no doubt that a pair of JLs costing around $6K could make a $1K pair of speakers sing as never imagined.
Anyway, back to your system, I suspect you'd get your best results by running your main speakers full range and sending simultaneous signal to the sub either via a second set of speaker cables from the amp or a second set of RCAs from the preamp via Y-adaptors. Then use the sub's volume, phase, and crossover controls to blend it (them) with your main speakers.
For blending sub(s) with a 2-channel music system, if you can't afford (or lack the space or floor strength) for a do-it-all sub such as a JL or Velodyne DD+ series, then go for the light'n'lively type of sub with a sealed enclosure, perhaps with passive radiator(s), lots of power, and fully continuous crossover, volume, and phase settings for the best blend with your main speakers.
