While maybe interesting on 6m (50 MHz), I'd expect the losses in twin-lead RF lines at 144 MHz and especially 400+ MHz (70cm) to be much higher. Honestly, twin-lead is really not made for anything that's beyond 100 MHz - the conductors are really (typically) just unbraided litz wire, which leads to a lot of conductive losses, and the fact that the geometry of window line is much less tightly definable than on coax means that at least to 70 cm, I'd expect some 30 m of twin-lead to look more like an antenna then a transmission line.
On the other hand, LMR-400 is a nice, low-loss medium. For 150 ft (my head sas that's 50 m in non-freedom units), I would expect some 4.4 dB in loss at 422 MHz – quite good, actually! For the 50 MHz (6m band) sine, I'd expect some, rule of thumb, 1.5 dB loss. So, while not zero, you'd need to do extremely well on the necessary matching, connection, weatherproofing and isolated mounting of the window line to not get more loss there than what you'd save at the low frequencies.
So, I'd just be happy about my (not really cheap!) LMR-400 line being in place, and I'd leave it untouched.
the loss on the line making it very hard to hear/receive certain signals.
I'd have my doubts: For microwave frequencies, i.e. > 300 MHz, we typically model noise to be dominated by 1. interference and 2. receiver noise. With narrowband receivers such as used by ham operators, the noise figures of receive amplifiers in that band should be low enough that losing a few dB on the cable doesn't matter – it does not make the interference-to-signal ratio worse, only the receiver-noise-to-signal ratio, and that's probably already not the problem.
For the 6m band, if I'm not mistaken, we model noise to be dominated by atmospheric noise, so that the cable loss doesn't matter at all! But I'm not an expert on these frequencies (they typically fall into what I'd usually call baseband…), so I'd have to defer to others there.
To know in what noise regime you are, maybe try with a low-power digital mode like FT-8 reception first. If that works well, you can use it to figure out whether your signal is actually close to the sensitivity of your receiver, or whether it's the interference that kills your reception. Good thing, that's probably "free to try", as you seem to have a computer!
Financially, trying with a slightly narrower preselection filter and an LNA might be cheaper than buying window line, cutting your expensive LMR-400, matching and connecting the window line and then, and that's the part where regrets happen if something goes wrong and water enters the coax, weatherproofing the connections. So, I'd start there!
