BEFORE:
I advise getting a sound meter and studying how to use it in various locations in your listening area. Then, move your speakers around a bit, keep checking, before adding tone controls. You will be ready, knowing what frequencies you want to primarily control. Tone controls are interactive, avoid aggressive use, best to know if you need general or specific adjustment.
AND, if/when you change speakers, you will be able to see how they interact in your listening area.
Tape loop as others have said.
You can also add a unit prior to the preamp,
You can use it's tone controls, use it's other features, send it's output optionally: use only one input in your preamp, (. i.e. tape monitor, aux, phono HIGH, any unused line level input) or into integrated amp, or directly into your amp if you have volume control somewhere.
I use Chase Remote Line Controler RLC-1
https://www.ebay.com/itm/NEW-Chase-RLC-1-Remote-Line-Controller-New-with-remote-manual-in-original-box/293098775817?epid=1424512686&hash=item443e0c6509:g:1AEAAOSwS1Nc5E7q
From my listening position, it gives me remote volume, and the feature I most love 'balance'. I find many recordings benefit from very minor balance adjustment. Amazing how much difference that can make.
It has tone 'bass' and 'treble', however, they change both channels together, so you cannot make a squeak of a difference left or right if that rare need exists.
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It is a separate discussion regarding the RLC-1's automatic 'LOUDNESS' compensation at low volumes.
Or ANY 'loudness' compensation. Be aware while auditioning any tone control unit, customers and/or salesman may have left it improperly engaged, a poor impression of an excellent unit might be experienced.
IMO, it is poorly understood, therefore it's loudness is engaged before beneficial, resulting in boosted bass at too loud volume levels. In fact, 'loudness' has been misunderstood, improperly engaged, almost always. It should have been named 'low volume eq'.
Loudness Setup: Use primary volume for your LOWEST focused volume listening from your dedicated listening position. Then, when listening at lower volumes (unlikely to be in primary listening position), as you lower the volume, LOUDNESS progressively boosts one or eigher end: most often bass, original Fletcher Munson Curve boosts both bass end and high end progressively as volume decreases.
Properly engaged, 'loudness' definitely moves low volume sounds from background noise to 'involving' music. When you lose bass or highs due to human ear's variable sensitivity, you lose both the fundamentals and the overtones.
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Keep in mind, as we age, we progressively lose the ability to hear highs, therefore, a 'loudness' curve boosting both lows and highs helps with that. RLC-1 boosts only the bass. This is why a reviewer of age discussing highs is laughable.
If you miss tone controls, as you age, you benefit from treble control more, and treble boost (narrow dispersion) is less likely to involve room interactions, boosted bass (very wide/omni-directional dispersion quickly gets magnified by room interactions, especially if 'loudness' is engaged too early.
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btw. RLC-1 has two sets of outputs. It was developed in the era of QUAD, so it has 'front' and 'rear' and 'fader'. I use the rear to feed an alternate amp setup, with interchangeable speaker wires via bananna plugs, to switch between tube amps and digital amps. EVERYONE chooses my 30 wp tube mono amps over my 320 wpc McIntosh solid state.
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btw. Check any tone control's signal to noise ratio, crosstalk, and channel separation. Highest, highest, highest, you want it to be 'invisible' in the audio chain except what you specifically want.
RLC-1's ratings are 105db; -110db; 100db. No one can hear any difference with it in or out of my chain, so I benefit from all it's features with no audible penalty.
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BTW, I sound like a Chase salesman, but I never owned Chase stock, and the company is long out of business. It is a wonderfully misunderstood unit.