Increasing bass on the Millett MAX:
In the order of effect on bass, here they are -
1. 12AE6 tubes. This is the number one thing that you can do to increase the bass slam. Gain is high on these, though, so you will be restricted in the volume travel - 1/4 to 1/3 turn for full volume with most phones.
2. Ensure that CA2, the cathode bypass electrolytics, is at least 1000uf. The MiniMAX uses Muse ES at 1000uf 16V - this is the best for bass at 25mm height. It depends on how your MAX has been built and what slot the PCB is in on the Hammond case. If you have 1-1/2" clearance, which is achieved by placing the PCB in the bottom slot, then you can go with Elna Silmic RFS II's. DigiKey has them at good prices - 1000uf is key, but voltage can be variable as long as you don't go under 16V. Again, you can't use Elna's if the PCB is in a higher slot in the case.
3. Be careful with the film cap bypasses in this position - CA9. Wima's are fine here, unless you were lucky enough to get Black Gate NX's. With Black Gate NX's, you need to leave CA9 blank. With other caps at CA2 - including the Muse ES - the Wima's (MKP10, 0.22uf) are ideal. However, other caps - especially boutique ones - at CA9 can actually reduce the bass. Vitamin Q's or Sonicaps in this position will cut the bass.
4. Typically, CA7 has been a Muse ES at 470uf 35V. This is the standard on the MiniMAX. It serves quite well for bass even for low impedance Grados, because Grado's bass peaks at 50Hz, not 20Hz. However, if you have other low impedance phones, you would benefit with more bass by going to 1000uf at those positions. If you have the extra clearance, go with Elna Silmic RFS II's at 1000uf. Voltage is critical in these positions, though, and must be 35V minimum. If you don't have the height and want the extra uf - then go with the Muse Fine Gold. There is a 1000uf 35V Fine Gold that is only 25mm (it's the same size as the ES at 470uf 35V). I offer the Fine Gold at 1000uf 35V as an option on the MiniMAX.
5. You can increase the bass with higher-rated resistors at RB14, but they will cut the detail as you go higher. In fact, the bass effect has more to do with lowering the cutoff frequency with lower-impedance headphones or simply rolling off the highs. A better, bigger capacitor at the CA7 positions is the better alternative, IMHO.
6. The film caps that bypass the CA7 position - CA8 - will not affect the bass at this position. Boutiques are fine and we have used Vitamin Q's in the past and use Sonicap Gen II's for the MiniMAX at this position now. The CA8 caps affect treble extension and highs' harshness, so more on them below.
Increasing Treble on the Millett MAX:
In order of effectiveness -
1. The old combination of CCS resistors that most people used in building the MAX was 1K and 10K at RA8 and RA9, respectively. These were not optimum, but thought to be a good compromise with the availability of typical-size V-D resistors. However, we have since found out that 1.15K and 11.5K are readily available and we have since made this the standard on the MiniMAX. In fact, I will send the CCS resistor pairs to you free if you send me your address. They make that much of a difference: distortion on the tubes is much lower with these resistors at the CCS positions. This change has the single greatest effect of improving the highs and taming any harshness.
2. Ensure that CA7 is an audio-quality, boutique capacitor. Either the Muse ES at 470uf 35V or the Muse FG (Fine Gold) at 1000uf 35V are your best bets if you have the 1" (25mm) height restriction. This has the second greatest effect at highs extension and harshness.
3. Ensure good quality bypass film caps at CA8. Boutiques are a good choice, here. Again, the standard on the MiniMAX used to be Vitamin Q's until their availability ran out. We currently use Sonicap Gen II's at 0.33uf 200V. Either is a great choice, but there are many others that you can use. Wima's have always been used as film bypasses on the Millett Hybrid, but in my opinion - they impart a bit of graininess at this final output positiion. I would stick with something name-brand, axial, and at a reasonable price. Auricaps are not too expensive at these sizes - 0.22uf should be a good target, but you can go higher (0.33uf, 0.47uf), but only slightly lower (0.18uf) is recommended. Just MHO, but I would steer away from the Wima-similar caps such as other box caps like the Wima's - orange drops, Roederstein, etc. They seem to have no effect at improving the highs or add graininess like the Wima's. Again, the Wima's are great at all the other positions on the PCB (CA3, CA6, CA9), but this one is most critical to the final sound. Voltage is not critical with any of these film caps. All you need is 35V and most film caps are 100V and higher.
4. 12FK6 tubes. These generally have the best highs, but unlike the 12AE6's with bass slam, it's not as much as the other items above.
5. The output resistors will have the final effect on the highs. They can either outright roll off the highs or add "sweetness." The Riken resistor is probably the best at this position, but is expensive. I would stay at 10R unless you're really interested in actually cutting the highs. Any higher and it really cuts the detail. 10R seems to be the best for tweaking. The Kiwame's are probably the best and are not too expensive, depending on the source. Stackpoles are next, but they will tend to roll off more than clean up the harshness (sweeten). This is not much different than what I originally wrote up there on the website, with the exception of mentioning the Riken's.
Good luck!
EDIT: Corrected some things ...
