I'm 71, have tried/learned several bass setups over the years.
I encourage tubes, and vinyl, reel to reel, so efficient speakers to reduce tube power needs. Entire system: darn good, not perfection, more money saved for more content to enjoy.
I go for darn good directional stereo bass for main listening,
listen to this, you will want directional bass.
https://www.amazon.com/Double-Bass-Niels-Henning-rsted-Pedersen/dp/B00000G62F/ref=asc_df_B00000G62F/?tag=hyprod-20&linkCode=df0&hvadid=312683822382&hvpos=1o1&hvnetw=g&hvrand=18279210393491330306&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9003512&hvtargid=pla-540093611693&psc=1&tag=&ref=&adgrpid=65958438881&hvpone=&hvptwo=&hvadid=312683822382&hvpos=1o1&hvnetw=g&hvrand=18279210393491330306&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9003512&hvtargid=pla-540093611693
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Amount of bass: avoid too much, seems cool at first, but detracts from your delightful mains imaging, and loses bass direction. Extend system as desired, but not too much, yet definitely aware when you turn them off.
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Very careful consideration of room/placement/control options in advance
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music system, medium sized room: pair of subs and delightful mains:
pair of self powered directional subs. When mains not trying for low bass, mains are smaller, easier to place, and, importantly, main amp can be lower power, easier to use/try tubes.
very strong magnetic control of sub's woofers, avoid unwanted delay/distortion, preserves direction. Perhaps servo control of sub needed, depending on sub size/design.
solve location of mains without subs, then find nearby directional location of subs, refine sub crossover and volume control.
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Sub remote volume control. Not common, or critical, BUT, remote control of sub volume is handy, especially home theater. Consider ease of use: internal sub amps with that feature, in-line volume control, home theater receiver easy menu access to adjust, exterior dedicated amp with remote volume. Individual remote volume control of pair of subs can make a difference.
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home theater, office (not big ones).
Single self powered sub, non-directional, added to smaller mains in home theater, or bookshelf in office. Same reason, mains can be smaller, easier placement.
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other considerations.
delightful mains, tweeter ear height, mid close to that, no ports.
ability to locate/move mains away from rear and side walls (less critical when they make less bass)
If need to move speakers back, forward only when listening, no spikes, heavy, i.e. sand/shot filled movable base, i.e. felt slide-able bottom.
16 ohm comments. my mains are 16 ohm, incredibly efficient. no matter what the math says about reducing the amps power, they are the loudest speakers I have ever owned, need no moe than 30 wpc (perhaps less, never tried). I think the other way: 8 and 4 ohm speakers increase the
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NEED for more power. Considering amp cost, size, weight, heat, large amps are generally to be avoided IMO. Especially if using or will try tube amp.
You won't avoid 8 ohms today, you can avoid 4 ohm speakers, and certainly, to use tubes, avoid inefficient speakers. Let the subs have their own power, remove bass needs from amp before it goes to mains, otherwise the mains will get the bass signal, they and amp will try and fail.