'substantially down' is a correct and usable term in written English.
You can use it when talking about a decrease in quantity, size, degree, or quality. For example, "The number of people attending the event was substantially down from last year."
Milk is substantially down in price".
Sky's audience was always going to be substantially down on last year's free-to-air coverage.
But hotels say that bookings are substantially down and traders complain that the promised boom just has not happened.
Currently, unleaded petrol and diesel both cost around £1.11 per litre, substantially down from the £1.50 per litre seen in 2012.
The scheme had some success crime levels are substantially down and some areas of shops and homes are smart and tidy.
That means that IDC now expects total shipments will hit 303.5m in 2014, compared to 315.1m in 2013 - which was itself substantially down from the 352m in 2012, and 361.5m in 2011, when the market peaked.
We also found that transcripts of three late biosynthetic genes were substantially down-regulated.
Gene expression changes in common include transcripts related to fatty acid synthesis and degradation, TAG synthesis, and a putative TAG lipase that is substantially down-regulated.
Moreover, JNK phosphorylation was substantially down-regulated in liver lysates of HS+MES-treated mice (Fig. 4C; pJNK).
The transcript levels of the two genes were substantially down-regulated by injection of gene-specific dsRNAs of the two trehalases.
The potential clinical significance of the NKp30 down-regulation is highlighted by its importance in recognizing melanoma cells, as suggested by the results described in Figure 5. On the other hand, CD16 was substantially down-regulated in melanoma patients (Figure 6) and did not directly bind to melanoma cells (Figure 5).
