The stock SEX plate choke is about 30 henries; the MQ upgrade is 50 henries. For comparable bass performance you need a series feed transformer with at least this much inductance. Such transformers are not impossible to build, but they are not very common either. To be fair, such a transformer should be compared to the MQ upgrade output transformer, not the stock Speco.
Edcor and Hammond both make less expensive series feed transformers with lower inductance, which might be very approximately compared with the stock plate choke/Speco output combination. (The Speco inductance can be anywhere from less than 10 to over 100 henries depending on the signal frequency and level, so it's hard to guess what series-feed output transformer inductance might "match" the stock parafeed combination.)
The reason for the inductance requirement is to prevent a low inductance from demanding too much current from the output tube in the bass. In parafeed, if the parafeed capacitor is properly sized, the impedance seen by the tube is maintained high and resistive to a lower frequency that a simple inductor of the same inductance as the plate choke, so a larger inductance could be required for a series feed transformer to match the low-frequency distortion performance of a parafeed arrangement.
As a rough comparison, the Magnequest DS-050 and the cheaper RH-40 are both rated 40mA at 40 henries. With 20% more turns on the primary they would have an 8K primary impedance, 60 henries inductance, and 32mA current capacity - ideal for a SEX amp with comparable performance. I can't find the exact details but these transformers are roughly the same size as the SEX amp power transformer, and weigh about twice as much as the stock output transformer and plate choke combined.
The other thing about series feed is that the output signal current flows through the power supply. That means two things - the power supply must have more filtering to maintain the same hum level, and the last filter capacitor must be higher quality (comparable to the parafeed capacitor) to provide a similar sonic quality.
There is no reason not to experiment with this comparison, other than cost and the mechanical difficulties of fitting different components. I would suggest the RH-40 as a good starting point - the lower plate load impedance is not so low as to make it unreasonable. Change the 47uF/450v power supply capacitor to something larger (390uF perhaps?) bypassed with a Solen 47uF/600v unit so as to maintain the midrange quality equivalent to the normal Solen parafeed capacitor.
An alternative would be to add an RC stage of power supply filtering to lower the voltage, and increase the quiescent current of the 6DN7 correspondingly, to optimize for the 5K load impedance. I'll work out the numbers if anyone actually plans to do this.
