These are the criteria for inclusion in my quiet tube-amp product categories.
Less than 10 watts. May or may not include a speaker -- if so, it's 12" or maybe 10", but certainly not 8" (though perhaps even 6" is ok if part of a speaker isolation cabinet). Tiny amps with tiny cabinets and tiny speakers, like a 5 watt Champ, do not count -- the goal is full-featured controls and a full-sized speaker, such as the Hi-Mu.
What would be really handy and forward-thinking is a rack-mount low-watt tube power amp with no controls, meant to be used with a MIDI-controlled preamp that contains compression, distortion, and eq, and other EQ-based or dynamics-based effects, but no time-based effects in the preamp. They make 100-watt rack mount tube power amps with no controls; they should do the same with 3-watt tube power amps.
Can drive a speaker directly, with no attenuator, or you can unplug the speaker and an attenuator (that is, dummy load) kicks in, for line-level out (which can be used for post-amp time-fx and EQ, before monitoring).
Input: power-amp signal. Output: high-level; can drive a guitar speaker. The unit soaks away some fraction of the power. May or may not have a line-level out, as well.
Input: power-amp signal. Output: line-level signal. May or may not have a speaker simulator circuit, such as a high-cut filter.
Input: line-level signal. Output: line-level signal.
Strive for amp emulation; that is, producing the dynamic response of a power tube, output transformer, guitar speaker, cabinet, and mic, and ideally, room response via post-amp time effects and EQ. They do not merely include primitive "speaker simulation" such as treble roll-off. They do not use a power tube. If it did, I would put it in the "recording amp" category.
May or may not have an attenuator attached. Contains a guitar speaker and mic.
Whether or not you use a modelling amp, amp simulator, or speaker isolation cabinet, I strongly recommend that you somehow include a saturating power tube in your rig, such as from the following devices. Hopefully, many more low-watt power-tube products will be designed soon, such as adding an EL84 in the middle of the DigiTech 2120 processor or Line 6 Flextone amp. I am especially looking for very low-watt power-tube products that can support both approaches: either driving a guitar speaker and mic, or using an internal dummy load and cabinet response filter. I would expect a simple, low-end, stripped-down power-tube saturation pedal to cost only $150 list, or less, and a simple, to-the-point rack amp with small power tube to cost only $300 list, or less, and a straightforward, modelling amp preamp/processor with small power tube and intelligent, fixed placement chain to cost only $1000 list. High-end models could be available too, but the first order of business is to put in the power tube, and take out the expensive distractions and 3rd-order irrelevancies. Most of these products, with a low-watt power tube and load, are absurdly overdesigned and thus overpriced, due to lack of focus on the central, simple, essential need of home studios. The point that home recording studio guitarists place first, is simply to *have* a power tube, at all -- not eliminating all solid state processing, or using point-to-point wiring, or supporting a variety of biasing modes, with 6 output jacks and footswitch control. That kitchen-sink vintage approach is not what most guitarists want. Strictly speaking, a power tube, output transformer, power supply, and case, and driver circuit, should only cost about $50 -- far less than $600.
Aside from price, my sort order and main priorities used for my table of low-watt power tube products are: first, must be able to drive a guitar speaker directly (because I don't trust dummy loads and cabinet response filters, which can sound fizzy and fake). Second, for when not using a guitar speaker, the coolest units are those that have a load and filter built in - to form a self-contained literal saturating tube amp in a box, with simulated speaker (hopefully, optional, so that you can choose to use a guitar speaker rather than full-range speaker for final monitoring). That's the main vision: a black box that takes a preamp signal, and outputs the sound of a saturating power tube directly interacting with a hard-driven guitar speaker, as heard by a microphone. Due to the convenience but imperfect sound of a load and filter, a unit that focuses on addressing the basic "literal tube amp in a box" vision should support both approaches: using an actual speaker and mic, or using an internal dummy load and cabinet response filter.
For a unit that cannot drive a guitar speaker directly, that's better than a plain vintage combo amp approach. Here's my sorting logic for authentic DI units (those that have a low-watt power tube): Strongly desire a dummy load. Moderately desire ability to drive spk. Slightly desire to have a cabinet response filter. Finally, attenuator is better than none, and then last of all, sort by watts, then price.
Thus for the following criteria:
Guitar speaker Out, Dummy load, Cabinet response filter
the sort order is:
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