It might be helpful, if Bob permits, at this moment to go a bit into Speaker History, with specific reference to that class of speakers loosely described as “Bookshelf”.
Back in the days of yore, speakers were largely not in any sort of formal Box, but typically mounted in a panel that may or may not be stand-alone, and had little or no ‘design’ to it other than, perhaps, aesthetics. Not all, of course, but most.
Along comes Edgar Vilchur and “Acoustic Suspension”. Now, we have small box capable of extended bass performance. Over time Vilchur (and his company, Acoustic Research) developed and introduced multiple-driver speakers with actual (passive) crossovers and other refinements. Rapidly, AR spun off competitors, who, in turn spun off yet more. And as most of these stayed in the Eastern Massachusetts area, this class of speakers tended to sound close to the same – and became the “Boston Sound”.
JBL and others, almost simultaneously, started to develop competing designs that were not sealed boxes, but were approximately the physical size. And, being both smart and intelligent competitors (not hardly the same thing), they played on the inherent weakness of acoustic suspension designs, being that they are power-hungry, and further exploiting the increasing use of dome mid and tweet drivers – also requiring more power than conventional designs. JBL, Cerwin-Vega, and others, then all California-based, developed this line in direct competition to AR, KLH, Advent, Boston Acoustics, etc. And, became the "California Sound".
The typical Acoustic Suspension speaker was inefficient (86 dB to as low as 78 dB), had a relatively smooth frequency response, dropping off sharply outside the design range, and were relatively small for the performance they gave. They also tended to have a very wide sound-stage due to the common use of domed drivers.
The typical competing design (AKA “Ported” for now) was much more efficient (90 dB+), a tad larger for the same frequency range, tended to be bass and treble-heavy with relatively reduced mid-range, and by avoiding domes for the most part, concentrated their sound within a relatively narrow cone. The roll-off at the extremes also tended to be far less sharp.
Now, Damping Factor and why it I focused on it in this discussion: A speaker that is bass-heavy will have an exaggerated response to the damping factor of any given amp. Now, let’s demonstrate exactly what it is – and this will take some small effort on a reader’s part:
• Obtain a small battery-powered electric motor such as would be used for a model car or boat.
• Spin that motor with your fingers. Note that it spins fairly easily.
• Now, short the battery connections to each other. Note that the motor becomes much harder to spin. Those of you old enough to remember slot-cars remember that this is how they braked.
• This same phenomenon is how the damping factor works on speakers.
The OP did mention being unhappy with the bass response on the (OEM) ST70.
Knowing that the speakers he referenced are 90% efficient:
• A 35 watt amplifier will give ~105 dB
• A 60 watt amplifier will give ~108 dB
• A 75 watt amplifier will give ~109 dB
• A 125 watt amplifier will give ~111 dB
All without clipping. All rounded to the nearest whole dB.
With a total spread of 6dB, not a whole lot, and with a given complaint about poor bass response, and with the inherent design of the speakers cited being relatively bass-heavy, I still suggest that the OP should consider Damping Factor in his choices moving forward. All other things being equal, the next consideration would be headroom for transient response.
With the choices offered from the VTA family being the 70 and the 120, and the incumbent being an older-model McIntosh MC275, the 120 is a better choice than the 70, neither is as powerful as the 275. BUT, given the exaggerated resale value of the 275, this situation is crying out for dual M125s. Which was not mentioned initially, and would be an improvement over all the other options at every level.
This on the essential given that the OP wants tubes, not solid-state.